Dallas Says That Any Change in Velocity Is Directly Proportional

Multiple Choice

Dallas says that any change in velocity is directly proportional to the time interval over which the change took place.Dana says that is true only when the acceleration is constant.Which one,if either,is correct?

A) Dana,because it is true only when the acceleration is constant.
B) Dallas,because we can define ax_,avg so that Δv_x = ax_,avgΔt.
C) Dallas,because ax_,avg always is equal to .
D) All the above are correct.
E) Only (a) and (b) above are correct.