新prep174

yunshishow

174.
Often major economic shifts are so gradual as to be indistinguishable at first from ordinary fluctuations in the financial markets.
A.    so gradual as to be indistinguishable
B.    so gradual so that they can be indistinguishable
C.    so gradual that they are unable to be distinguished
D.    gradual enough not to be distinguishable  
E.    gradual enough so that one cannot distinguish them
答案A，prep里说C中的unable to be done错误，弱问这个结构为啥错了？

yunshishow

谢谢哈，明白了~~~

houhaibo528

A is unable to be done by B,
this structure is redundant, becoz it equals to " B is unable to do A "