The prefix "ex_-_" means out of or away from and the word “change” means to make or become different, so when you “exchange” something, you give away one thing and get something different in return for it. This is most similar in meaning to “trade.” To provide further help, “study” means to review and practice material to learn it; and “wander” means to walk around without a destination.

The prefix "semi-" usually means half, so we can assume that a “semicircle” is half a circle. To provide further help, a three-dimensional circle is called a “sphere;” “parallel” means running alongside, having the same slope; “slope” means slanted ground; “gradient” means slope. In addition to being terms we use in English, these are also all mathematical terms.

To “surpass” is to go beyond or to become better than someone or something. “Exceed” also means to go beyond the limit of something or to become better than someone or something. Look for prefixes like "sur-" and "ex-"where "sur-" can mean above and "ex-" can mean out of. To further help you, “recede” means to go back, so you can say waves “recede” on the shore.

The prefix "under_-"_ means below or beneath, so the word “underdeveloped” means not grown enough, not developed enough. To provide further help, “developed” means grown; “beneath” means under.

The prefix "im-" means not, as in the words "impossible," "impolite," and "improbable." So, "impermanent" means temporary or not permanent, and "temporary" is the correct answer.

"Subterranean" means underground: It contains the prefix ­"sub_-_," which means under or below, and the root "terra," which comes from the Latin word for earth. So, "underground" is the best answer, because it is closest in meaning to "subterranean."

The prefix "ne-" often indicates a negative word. "Negate" most nearly means to invalidate or to disprove something. "Validate" is an antonym to "disprove."

“Dismount” means to get down. It is commonly used when speaking about “dismounting" from horses, as in to get off your mount. The prefix "dis-" is a negation prefix and the suffix "-mount" refers to something you sit on.

The suffix "-act" means to do, to act, to respond. The word “react” means respond to something. To provide further help, “writhe” means twist in pain; “egress” means exit, way out; “akin” means similar to; “variegated” means different looking, multicolored.

"Extraterrestrial" means alien when used as an adjective, and an "extraterrestrial" is is an alien. So, "alien" is the correct answer. To help you understand the formation of words, consider that "terrestrial" means from the earth and in this context, the prefix "extra-" means something other than. So an extraterrestrial is something from somewhere other than the earth. To help you further, "extraordinary" means something other than ordinary, "ordinary" means normal, a "foreigner" is someone from another country, and a "citizen" is a member of a nation.