Great as Newton was, many of his ideas _______ today and are being modified by the work of scientists of our time.

A．are to challenge

B．may be challenged

C．have been challenged

D．are challenging

合理确定工程造价，就是要求工程造价的确定要具有（）。

A.科学性

B.预见性

C.经济性

D.合理性