Divisibility by three and nine

This task is about whether numbers are divisible by three or by nine. 
Answer the following questions without using a calculator. 

Question

a)  Is 25,396 exactly divisible by 3? (Choose one)

    • YES

    • NO

Explain how you could decide this without having to do the division.

Question

b)  Is 4,776 exactly divisible by 9? (Choose one)

    • YES

    • NO

Explain how you could decide this without having to do the division.