Which expression has a positive quotient?

Can someone check my answer? you

Which of the following is most likely the next step in the series? a3z, b6y, c9x, d12w, е15v, f18u

How do i start to solve? would appreciate a walk-thru! a bird sitting 16ft above the ground in an apple tree dislodges an apple. after how many seconds does the apple land on the ground? (assuming that no branches will interfere with it's fall)