细胞内含Birbeck颗粒的是（）
A.角质细胞
B.成纤维细胞
C.基底细胞
D.朗格汉斯细胞
E.梅克尔细胞
参考答案：D
Today Chinese is becoming more and more popular. It _____ in many schools around the world. [ ]
A. teaches
B. has taught
C. is taught
D. is teaching
錾子楔角在錾削一般硬度的金属时可取（）
A、20~30
B、30~50
C、50~60
D、60~70
