【正确答案】 C

【答案解析】though意为“虽然”，例如：I think the poem is very good，though it's written by anobscure poet．我认为这首诗很好，尽管它出自一位默默无闻的诗人之手。unless作“如果不，除非”讲，例如：You can’t appreciate English poetry unless you understand its rhythm．你若不懂英文诗的韵律，就不能欣赏英文诗。as意为“像……一样”，例如：Just as onecan not learn to swim while standing by the pool，SO one cannot learn to sketch and expressgraphically only by reading about it．正像人们不可能站在游泳池边学游泳一样，人们也不可能仅仅通过阅读有关制图的书就学会制图和用图来表达自己的思想。since作“由于”讲，例如：Since she has apologized，I am content．既然她已经道歉了，我就满足了。