1.“I don’t like to travel. ”“Have you ever _____ in an airplane?”
a. flying b. flew c. flowed d. flown
2.The center of gravity of the human body ______ behind his joint.
a. located b. locating c. to locate d. is located
3.“Today is very cold, isn’t it?” “Yes, the river is _____ over.”
a. freezed b. freeze c. frozen d. freezing