What are the essential features of romance in the medieval English literature？

What should the tourists do to avoid illness during the trip？

They should take some essential ______.

必需氨基酸(essenti alamino acid，EAA)

Soil that is rich in organic matter______.

A．has nothing nutritious added to it

B．is essential for producing fruit and vegetables

C．contains unused vegetable matter

D．has had chemicals and fertilizers added to it

The author believes that man's "pursuit of happiness" in America has ______.

A．been essentially wrong from the very beginning

B．had disappointing results thus far

C．produced a finer, braver kind of modern man

D．fulfilled the hope of the American rebels of 1776

To many web-building spiders, most of 【C1】______ are nearly blind, the web is their essential window 【C2】______ the world: their means of communicating, 【C3】______ prey, meeting mates and protecting themselves. A web-building spider 【C4】______ its web is like a man marooned on an island of solid rock, totally out of touch and destined to starve to death. So 【C5】______ is the web to an orb-web spider's survival that the animal will continue to construct new webs daily 【C6】______ it is being starved.

For 16 days the 【C7】______ spider builds completely normal webs. Then, as the animal gets scrawnier, it constructs a wider-meshed web using 【C8】______ strands. Such webs would only trap larger prey, which is more 【C9】______ from the perspective of a starving spider.

The spider 【C10】______ energy by recycling web protein. It simply eats its own web each evening and reuses it to manufacture 【C11】______ silk. In studies with radioactively labeled materials, it 【C12】______ that 95 percent of web protein reappeared in 【C13】______ web. Most of the energy needed for web-building is used in walking over the strands as they are laid down.

Scientists are 【C14】______ by the adaptability of the spider's highly preprogrammed brain, which is larger 【C15】______ its size than the brain of any other invertebrate. If web-building is interrupted, or if some of the 【C16】______ strands are destroyed, the spider 【C17】______ retraces its steps to see where the web is 【C18】______ and then finishes building a 【C19】______ web. One spider will even finish building the incomplete web of 【C20】______ .

【C1】

A．them

B．whom

C．that

D．which