Multiply, and Then Simplify If Possible $( \sqrt { 7 } - \sqrt { 3 } ) ^ { 2 }$

Multiple Choice

Multiply, and then simplify if possible. Assume all variables represent positive real numbers.
- $( \sqrt { 7 } - \sqrt { 3 } ) ^ { 2 }$

A) $21 - 2 \sqrt { 21 }$
B) $10 - 2 \sqrt { 21 }$
C) $10 + 2 \sqrt { 21 }$
D) $4 - 2 \sqrt { 21 }$