Often major economic shifts are so gradual as to be indistinguishable at first from ordinary fluctuations in the financi

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
想知道选项的错误,答案是说不能用be able/unable to be done,我想知道是不是因为able只能用于人,所以没有被动语态?