胰高血糖素的主要生理作用是( )。

A．加速肝糖原分解及糖异生作用，使血糖升高

B．促进肝脏分泌胆汁

C．使血液中游离脂肪酸浓度减少

D．增强心肌收缩力，使心率加快，心输出量增加

—It is very important for young people to learn _____ useful foreign language.

—I know that, so I am working hard at English. [ ]

A. a

B. an

C. the

D. the