During that time, Mr. Li was wondering why she dropped chemistry and ____ medicine.

A．took over

B．took out

C．took up

D．took off

2011年1月11日12时歼20在成都实现首飞，该机可以有效避开雷达的探测，秘密之一在于它的表面有一层特殊材料，这种材料能够______（选填“增强”或“减弱”）对电磁波吸收作用．