Question

choose the appropriate auxiliary verb and fill in the blank
the first innings____(was/were)very interesting.​

Answer 1

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The first innings was very interesting.

Answer 2

The blank can be filled with the word 'was'.

So, correct option is 'was' and correct sentence will be:

The first innings was very interested.

There is a very interesting thing that inning and innings both can be used as singular.

In baseball and cricket, singular is used differently. But as first is used so it denotes it's singular so was is the better option.