Examination compositions, together with most business letters and government reports, are the main situations ____ formal language is used.

A．in which

B．on which

C．of which

D．for which

当差额投资内部收益率>社会折现率时，较优的方案是（）。

A、投资大的方案

B、投资小的方案

C、折现期长的方案

D、折现期短的方案