Dallas Says That Any Change in Velocity Is Directly Proportional $\Delta$

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 a_x,avg so that $\Delta$ v_x = a_x,avg $\Delta$ t.
C) Dallas,because a_x,avg always is equal to $$\frac { a _ { x , i } + a _ { x , f } } { 2 }$$ .
D) All the above are correct.
E) Only (a) and (b) above are correct.