Average acceleration is defined as:

.

In words, this definition indicates that the acceleration is equal to the rate at which the velocity is changing. Remember that a velocity can change in two different ways:

1. The magnitude of the velocity can change (i.e., the object can speed up or slow down).

2. The direction of the velocity can change (i.e., the object can turn).

You indicated that the acceleration at point D is zero. This means:

1. The magnitude of the velocity is not changing, i.e., the object is not speeding up or slowing down. Is this true?

2. The direction of the velocity is not changing, i.e., the object is moving in a straight line. Is this true?