Since the propensity to migrate has persisted in every epoch, its explanation requires to be ______ any particular period of time.

(单选题)—Which of the two subjects do you like, P.E. or music?—_____ (单选题)Lucy left with a smile on her face, ______. (单选题)All of us feel surprised that ______ a little boy can eat __ (单选题)He is so shy that he ______ in public. (单选题)—Do you know if he ______ back tomorrow?—I think he will com (单选题)He used to ______ in the morning, but now he is used to ____ (单选题)There is ______ “h” and ______ “u” in the word “hour”. (单选题)What is the main idea of the passage? (单选题)The author quotes the case of Albert Einstein to illustrate (单选题)A sound scientific theory should be one that _________.