“Surf,” when used as verb, means “ride on the crest of a wave, typically while riding on a surfboard.” So, one surfs on the surface of water, and we need to pick out an answer choice in which the first word is an activity, and the second word is the environment in or surface on which that activity typically takes place. While “ski is to snow” and “swim is to lava” might each look like a potentially correct answer, one does not usually swim in lava, and would not be advised to, so “swim is to lava” cannot be the correct answer. However, one can ski on snow, just as one can surf on water, so “ski is to snow” is the correct answer.

The word "secondary" refers to something that happens second, the second in a list of something. So, to solve this analogy you have to determine which one of these words means something that happens first, the first in a list of something. The correct answer is therefore "primary." To provide further help, "humorously" means done with humor; "universally" means by everyone, always relevant; "tertiary" means happening third; "unnaturally" means done unusually, done in a way that is not natural or normal.

The edge of "land," where the "land" meets the ocean, is generally called the "coast." So, to solve this analogy you need to figure out which of these words means something like the place where one nation ends, the line that divides one nation from another nation. The correct answer is "border." To help you "nation" is basically another word for "country."

You find clouds in the sky, so we're looking for a noun that describes a place where you might find fish. Of the three answer choices describing environments ("sea," "land," and "air") "sea" is the best answer, as it's where one is most likely to find fish.

Since a bed is the object used for the primary function of a bedroom (sleeping), we need to find the object used for the primary function of a restroom. The primary function of a restroom is to relieve oneself, so the correct answer is "toilet."

A "whale" is a large animal found in the "ocean. " So, to solve this analogy you need to determine where a "comet" is found. Well, a "comet" is a relatively small object found in space that orbits the sun. So, the correct answer is "space."

The ceiling is the upper limit of a room and the floor is the lower limit of a room. Since the sky is the upper limit of our environment, our answer must be the lower limit. The ground is the lower limit of our environment, therefore "ground" is the correct answer.

Since the relationship between daughter and mother is a same-gender child-parent relationship, we must find the answer that completes the same-gender child-parent relationship where the parent is the father. Since a son is a male-gender child and a father is a male-gender parent, the correct answer is "son."

"Peak is to bottom" compares two words that describe opposite ends of an object, its the highest and lowest points, so the two words have opposite meanings and are antonyms. "Large is to small" describes the relative size of two objects, not relative positions on them or sides of them. "Grade is to homework," and "tame is to domesticated" do not describe the highest and lowest point of an object or consist of pairs of antonyms. Both "top is to base" and ""flat is to bumpy" consist of pairs of antonyms, but only "top is to base" describes the highest and lowest points of an object, so it is the better answer choice and the correct answer.

Since an astronaut flies a space shuttle as part of his or her profession, we need to find a relationship where an individual uses a particular vehicle as part of his or her profession. Since only a firefighter would use a fire truck as part of his or her profession, the answer is "firefighter is to fire truck."