Gloria: In basketball play, referee is important.
Correct: In basketball, a referee is important.
EXPLANATION:

The indefinite article "a" refers to a particular or certain one of a class or group thing: a man; a chemical; a house.

Gloria: I improve my skill in test.
Correct: I improve my skill in answering tests.

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Hello, Gloria! Thank you for submitting your homework. Great work! Have a great weekend!

-Teacher Isabel