However difficult English is,I will learn it well.

B的话应该改成NO matter how difficult English is.你的B项is位置放反了.
再问： 空格, he would have been able to pass the exam. C.Had he studied harderD.Should he study harder 为什么D不行 __no air ,there would be no sound. 为什么是were there 不是was there
再答： should he study harder这是立足于现在，对将来的虚拟。而题干是would have been这是对过去的虚拟，所以时态用的是过去完成时。因为存在空气，所以是虚拟假设不存在空气，用were there，不用was，was不是虚拟。
再问： 只要是虚拟假设there都要加were吗
再答： 1.were表虚拟是跟现在事实相反的虚拟，跟there没关系，比如： If i were you, i would go to Beijing university.如果我是你（这是对现在的虚拟，因为我不可能是你），我就去北大。 If there were 3 apples, what would you do?如果有3个苹果，你会怎么办?(实际上没有3个苹果，只是虚拟)。 2.请注意，were表示虚拟的时候，不能用was替代，比如： Were he a teacher, his mother would be so happy. √ 如果他是老师，他的妈妈会多高兴啊。 Was he a teacher, his mother would be so happy. × 这是错误地表达。 3.另外if.+ 主语+were...的虚拟表达= Were +主语+...的表达（if省略了）。