but的沉思（能否分享be动词？），貌似是og上的矛盾....

    9. Sunsports, vortices of gas associated with strong electromagnetic acitivity, are visible as dark spots on the surface of the Sun but have never been sighted on the Sun's poles or equator

       B are visible as dark spots that never have been sighted on the surface of the Sun

       C appear on the surface of the Sun as dark spots although never sighted at

       D appear as dark spots on the surface of the Sun, although never having been sighted at

       E appear as dark spots on the Sun's surface which have never been sighted on

答案选A  解释是
            This sentence clearly and correctly draws a contrast betwwen where sunspots are found and where they are not.

  15.  The Iroquois were primarily planters, but supplementing their cultivation of maize,squash ,and beans with fishing and hunting

     B and had supplemented   C and even though they supplementd     D although they supplemented    E but with supplementing

 答案选D    而对于A的解释是The construction using supplementing fails to support the intended meaning of the sentence

      问题是，比较两句，发现都是 are ..... but。。。的形式， 不同点在于，第一个句子后but加出了第二个动词，对了，而第二句句子没有加出而是延续，这是否就说明but只有在补出动词的情况下才能延续起转折，否则不行？

自己顶

but supplementing their cultivation of maize

不通啊，没有谓语动词...