Susan's math teacher assigned x homework problems on Monday. On Tuesday, she assigned 12 more problems. Over the two days, a total of 21 homework problems were assigned. Which equation could be used to find x, the number of problems assigned on Monday?

A.
x - 12 = 21
B.
12x = 21
C.
x + 12 = 21
D.
21 + x = 12
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