表皮干细胞约占表皮基底细胞的（）。
A.10%
B.20%
C.5%
D.2%
E.30%
参考答案：A
His parents were worried that he ______ too much time chatting online. [ ]
A. spent
B. cost
C. paid
D. had
结合所学知识和原理分析材料并回答问题。运用所学知识分析下列命题：
