什么是主振动？两个主振动的合成是否为简谐振动？是否是周期运动？

Algebra and math.
B. English-language arts and mathematics.
C. Math and reading.
D. English and reading.

A. 40
B. 45
C. 50
D. 60

A. Study for some profession.
B. Attend a medical school.
C. Stay in business.
D. Sell his shop.