现存人类最早的文字是哪里、谁发明的、什么文字？
参考答案：两河流域、苏美人、楔形文字
心肌中主要存在的肌酸激酶同工酶是（）
A.CK-MB
B.CK-MM1
C.CK-MM3
D.CK-BB
E.CK-MM2
Our teachers always give us good advice. We should follow __________. [ ]
A. what they say
B. what they said
C. which they say
D. which they said
