[单选题]
07.17: A toy car moves along the x-axiy4 n v7jpj15svs according to the velocity versus 5v lc.fx; y6. xupl:ajtime curve shown to the right. Wfl jc5x;xp:v. .lua y6hen does the car have zero acceleration?
A. at 2 and 4 seconds
B. at approximately 3.0 seconds
C. at approximately 3.3 and 5.1 seconds
D. the acceleration is always zero
E. at no time