15. When we use the Doppler shift to study the motion of a star, there is an uncertainty in the inferred motion through space because: A. we need to make measurements at two different times to detect the shift at all, and the star's motion may itself have changed over that period of time B. the Doppler shift can only tell us how fast the star is moving, but not distinguish whether it is moving towards or away from us C. we only get the line-of sight motion, not the sideways component D. because of the exotic composition of the stars, the lines we see in the spectrum are often unknown, so the Doppler shifts are inevitably quire uncertain