一道ASSUMPTION题

X-ray examination of a recently discovered painting----judged by some authorities to be a self -portrait by Vincent van Gogh revealed an underimage of a woman's face.Either the portrait now seen on the surface of the canvas.Because teh face of the woman in the underimage also appears on canvases van Gogh is known to have painted, the surface painting must be an authentic self-portrait by van Gogh.

The conclusion above is properly drawn if which of the following is assumed?

A   if a canvas already bears a painted image produced by an aritst, a secon aritst who uses the canvas to produce a new painting tends to be influenced by the style of the first artist.

B   Many pained canvases that can be reliably attributed to van Gogh contain underimages of subjects that appear on at least one other canvas that van Gogh is known to have painted.

C   Any painted canvas incorrectly attributed to van Gogh would not conrain an underimage of a subject that appears in authentic paintings by that arist.

D  A  painted canvas cannot be reliably attributed to an artist unless the authenticity of any underimage that painting might conrain can be reliably attributed to the artist.

E  A painted canvas cannot be reliably atributed to a particlar aritst unless a reliable X-ray examination of the painting is performed.

答案为C ，是不是因为C 是题目中的逆否表达，才所以是对的？我怎么觉得BCD差不多呀？

BCD 都很像，  但是假设不能过分，  否则取非不能削弱。

用取非削弱的方法 （主句取非） 很清楚就看到了B非，D非，不严格削弱，C严格削弱