预防佝偻病最有效和最经济的措施是
A．母乳喂养
B．加服钙剂
C．多晒太阳
D．加服维生素
E．添加辅食
参考答案：C
先马走
— ___ do you go to the zoo?
—Once a month. [ ]
A.How
B. What
C. How often
D. Why
