In the following passage some of the words have been deleted. Read the passage carefully and select the correct answer for the given blank out of the four alternatives.
One afternoon, a dozen Arizona State University students (1) __________ to spend the morning cutting cardboard, taping fans and assembling filters in an effort to (2) ______ 125 portable air purifiers for local schools. That (3) ______ morning, staff members at a homeless shelter in Los Angeles were setting (4) _______ 20 homemade purifiers of their own, while in Brookline, Massachusetts, another DIY air purifier was whirring quietly in the back of a day care (5)________ as children played.
Select the most appropriate option to fill in the blank No. 4.