【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