角质层由多层扁平的角质细胞组成，细胞完全角化无生命。
参考答案：对
Every means _____ tried but without much result. [ ]
a. has been
b. have been
c. are
d. is
"商品"与"顾客"两个实体集之间的联系一般是【 】
A．一对一
B．一对多
C．多对一
D．多对多
