Design Momentum Conservation Experiments Practice Test

You plan to verify momentum conservation for two carts colliding on a low-friction track using a video camera with analysis software (instead of motion sensors). You also have a meter stick and a balance. Which procedure best supports calculating $v_{1i}, v_{2i}, v_{1f}, v_{2f}$ from the video?

In designing an investigation to verify $p_{\text{before}} \approx p_{\text{after}}$ for cart collisions on a track, you want to minimize external forces. Which choice lists a controlled variable that should be kept as constant as possible across trials to improve the validity of the momentum conservation test?