耶克斯和多德森认为，当学习任务比较复杂而且困难时，可达到最佳水平的学习动机应该是（）。

A．高

B．低

C．较高

D．较低

The entry point tissue in the very front of eye functions as the window ______ all light has to pass on its way.

A．which

B．that

C．across which

D．through which