【单选题】

Few mathematicians are ------- their futures on finding any such proof; instead, their efforts are focused on finding good, but not -------, solutions for most cases, a field of study aptly known as approximation theory.
A．(A) betting … perfect

B．(B) developing … convenient
C．(C) pinning … substandard
D．(D) destroying … exact
E．(E) establishing … vague