_______ in 1636, Harvard is one of the most famous universities in the United States.

___ by thick bamboo and high forest, the ancient city hadn’t been discovered until the 1990s.

If ____ in, viruses can result in illness or even death.

A sheep_______on this kind of special grass usually grows much faster than_______on ordinary (普通的).

______ down on the ground, he struggled to his feet and held up his head, _______his enemy.