I’m afraid I can’t attend your wedding ceremony, but thanks for the invitation_______.

--___do you clean the reading room? ---Every two or three days.

Nowadays the roles of husband and wife are not as ______ defined as before, especially when both parents work and earn money for the family.

______ acupuncture developed, the simple “bian stones” were replaced first by stone  and pottery needles, and ________ by metal needles.

The car is environmentally friendly because it uses electricity instead of gas. _____, it is less expensive.