The simplest form of 1 \minus{} \frac{1}{1 \plus{} \frac{a}{1 \minus{} a}} is:
(A)\ {a}\text{ if }{a\not\equal{} 0} \qquad
(B)\ 1\qquad
(C)\ {a}\text{ if }{a\not\equal{} \minus{}1}\qquad
(D)\ {1 \minus{} a}\text{ with not restriction on }{a}\qquad
(E)\ {a}\text{ if }{a\not\equal{} 1}