当丙烯腈含量低于7%时，丁腈橡胶就丧失了（）性。

A.耐寒性；

B.耐油性；

C.电性能；

D.机械性能。

Scientists from different countries are working together to _____ a good way to solve the problem. [ ]

A. keep up with

B. meet up with

C. catch up with

D. come up with