几道sc提

几道sc题
1，although schistosomiasis is not often fatal, (it is so debilitating that it has become an economic )drain on many debeloping countries.

a, it is do debilitating that it has become an economic
b, so debilitating is it as to become an economic
对于括号内的部分，哪个更好？
答案是a .但我认为b 好像也不错，为什么？

new hardy varieties of rice show promise of producing high yields without the costly (requirements of irrigation and application of commercial fertilizer by earlier high-yielding varieties)

a, requirements of irrigation and application of commercial fertilizer by earlier high-yielding varieties.
b, requirements by earlier high-yielding varieties of application of commercial fertilizer and irrigation
c, requirement for application of commercial fertilizer and irrigation of earlier high-yielding varieties
d, application of commercial fertilizer and irrigation that was required by earlier high-yielding varieties
e, irrigation and application of commercial fertilizer that were required by earlier high-yielding varieties

答案是e ，为什么直接用by 短语不能同时修饰requirement...... and applicaion......而只能修是最后一部分？
that 从句就能同时修饰前面的两个并列短语么？

3， in an effort to reduce thier inventories , italian vintners have cut prices ；their wines (have been prices to sell ,and they are)

a, have been priced to sell ,and they are
b, are priced to sell ,and they have
c, are priced to sell, and they do
d, are being priced to sell , and have
e, had been priced to sell ,and they have

答案是c
首先，我想知道后各分句中are priced to sell, and they do究竟是要表示什么样的意思
其次，在动词的省略形式中是不是只有一般过去时，一般现在是，能用省略形式？
现在完成时，过去完成时，现在进行时，过去进行时，不能只用第一助动词代替？

4，asrtronomers at the palomar observatory have discovered a distant supernovs explosion ,(one that they believe is )a type previously unknown to science.
a ,that they believe is
b, that they believe it to be
c, they believe that it is of
d, they believe that is
e, they believe to be of

答案是e ,为什么a,b, 中的that要省略？
one 引导的是什么成份？