Alternating current cannot be measured by DC ammeter because
A. Average value of current for complete cycle is zero
B. AC changes direction
C. AC cannot pass through DC ammeter
D. DC ammeter will get damaged

Hint:-We know that the devices for the measurement of alternating current and alternating voltage are different from the devices for the measurement of direct current and direct voltage.

Complete step-by-step solution:
An alternating current is a periodic current which means it repeats the same cycle after a finite period of time. It changes its polarity periodically which means it changes its polarity after a finite period of time. In the first half cycle, the alternating current has the positive polarity while in the second half cycle, the alternating current has the negative polarity. The highest numerical value is the same in both the polarities. Thus, the average value of the alternating current for a cycle is always zero.
A direct current is a constant current which has zero frequency. The direct current is not periodic. The direct current does not change polarity after the same period of time. Thus, the average value of the direct current is never equal to zero.
So, AC ammeter is used to measure the alternating current. DC ammeters can measure constant current i.e. direct current.Hence, alternating current cannot be measured by DC ammeter because the average value of current for the complete cycle is zero.
Therefore, option A is correct.

Note:- We have to keep in mind that alternating current has a frequency and direct current doesn't have the frequency. The alternating current reaches zero once in the cycle while the direct current is a constant current. So, average values of the alternating currents are always zero while the average values of the direct current changes according to the value of direct currents.