Factor the Expression $a ^ { 2 } - 4 a + 4$

Multiple Choice

Factor the expression. $a ^ { 2 } - 4 a + 4$

A) $( a + 2 ) ( a + 2 )$
B) $( a - 2 ) ( a + 2 )$
C) $( a + 4 ) ( a + 1 )$
D) $( a - 2 ) ( a - 2 )$
E) prime