肌肉中氨基酸脱氨基的主要方式是

A．L-谷氨酸氧化脱氨基作用
B．嘌呤核苷酸循环
C．联合脱氨基作用
D．鸟氨酸循环
E．转氨基作用

In our class, when the bell rang and the teacher closed his book, it was a（）for everyone to stand up.

A．signal

B．chance

C．mark

D．measure