Which of the following is/are true?

(i) if object's acceleration is constant then it must move in a straight line
(ii) if object's acceleration is zero, then its speed must remain constant
(iii) if an object's speed remains constant then its acceleration must be zero

A. (i) and (ii) only
B. (i) and (iii) only
C. (ii) only
D. (iii) only