Finding the number of $n$-words which can be formed under the given conditions.

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Consider all $n$-words from the alphabet {$0,1,2,3$}. How many of them have an even number of :-$\\$
$(a).$ zeroes $\\$
$(b). $ zeroes and ones $\\$
Source : Problem Solving Strategies by Arthur Engel$\\$
Note : We have to answer for both parts separately. $\\$
I am stuck in this problem as to how to approach, so all I am asking from you guys is to give me any sort of approach/ strategy.