Alternating current can not be measured by D.C ammeter because

Average value of current for complete cycle is zero
A.C. changes direction
A.C. can not pass through D.C. Ammeter
D.C. Ammeter will get damaged.

Correct answer: Average value of current for complete cycle is zero

Solution

An alternating current (A.C.) changes direction periodically, resulting in an average value of zero over a complete cycle, which makes it impossible for a D.C. ammeter to provide a meaningful measurement.

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