Factor the Composite Number 21 into the Product of Prime $7 \cdot 3$

Multiple Choice

Factor the composite number 21 into the product of prime numbers. For example: 30 = 2 * 3 * 5.

A) $7 \cdot 3$
B) $21 \cdot 1$
C) $3 + 18$
D) $19 + 2$
E) $1 \cdot 21$