727. The demand for airplane mechanics is expected to grow about ten percent a year in the next decade, largely because new federal rules calling for major modifications to older planes and because the airlines are adding hundreds of new jets.
(A) new federal rules calling for major modifications to older planes
(B) new federal rules, which call for major modifications to older planes
(C) new federal rules call for major modifications to older planes
(D) major modifications to older planes called for by new federal rules（C）
(E) major modifications to older planes are called for according to new federal rules
选 c, 为什么呢？ | 
