煤在重介质旋流器中运动时，作用于煤上的力主要有（）
A.重力和向心力
B.重力和阻力
C.重力和离心力
D.离心力和阻力
参考答案：C
The nurse is teaching a female client with osteoporosis about her prescribed diet. Which of the following foods is the best source of calcium（）
A. 1 cup of low-fat yogurt.
B. 1 cup of skim milk.
C. 1 oz of cheddar cheese.
D. 1 cup of ice cream.
