经生
参考答案：
魏晋时期出现，代人抄书以此谋生。
As a student, you should do your duty and put your heart into study ______.
[ ]
A. at one time
B. at all times
C. at a time
D. at times
以非兽药冒充兽药或者以他种兽药冒充此种兽药的为假兽药。
