单项选择题 The unit of (measurement) known as foot (has) originally based on the average (size) of the human (foot).

单项选择题 (If) the experiment succeeds or fails, it will (provide us with) (valuable) experience, which is essential to (improving) our future work.

单项选择题 His parents are (worried) (because) they haven't heard (of) their son (in) more than two months.