Question

完形填空最有一题为什么选can't而不是can

完形填空
Mr. Brown lives in a city. He 1 a small shop in the middle of city, and he sells(出售) pictures in it.
They're not expensive, 2 some of 3 are very nice.
One day, a woman came 4 the shop and looked at one picture for a few 5 . It's a picture 6
horses in a field. Then she asked, " 7 is this picture?"
"Twenty dollars." Mr. Brown answered.
"It's 8 expensive. Can I 9 you two dollars for it?" said the woman.
"Two dollars? But you 10 buy the canvas(油画布) with two dollars."
"Oh, but it isn't clean now," the woman said.
( )1. A. has
( )2. A. so
( )3. A. they
( )4. A. on
( )5. A. hours
( )6. A. on
( )7. A. How much
( )8. A. no
( )9. A. give
( )10.A. can B. have
B. because
B. them
B. in
B. minutes
B. with
B. How many
B. not
B. let
B. can't C. had
C. and
C. their
C. into
C. weeks
C. of
C. How old
C. too
C. have
C. are D. is
D. but
D. it
D. on
D. years
D. to
D. How
D. isn't
D. make
D. aren't
找了一个原文及翻译
Mr. Brown has a small shop in the middle of the city , and he sells pictures in it . They are not expensive , but some of them are very nice. One day,a woman comes into the shop and looks at a lot of pictures,Then she takes Mr. Brown to one of the the them and asks."How much is this picture?" It is a picture of horses in a field."Twenty dollar," Mr Brown answers."It is too expensive .I can give you two dollars for it,"says the woman."Two dollars? But you can buy the canvas(油画布）with two dollars." "Oh, but it isn't clean now,"the woman says.
布朗先生在市中心有一家小店. 他是卖画的. 这些画都不是很贵,但有一些确是十分精美.一天,一位妇人到店里来看画. 然后她带布朗先生到其中一幅画前并问"这副画多少钱?" 哪是一幅画着田野里的马德画. "20美元," 布朗先生说到. "这太贵了, 我最多给你2美元," 妇人说到. "2美元? 你用2美元只能买到油画布!" "是啊!但是现在的它并不干净啊!" 妇人嚷道.

Answer 1

解释如下
这个完形填空的场景是在讨价还价。
其实看这句
"Two dollars? But you 10 buy the canvas(油画布) with two dollars."
but出现，一般前后意思相反，因此前面是Two dollars,后句必然否定Two dollars,也就是can't buy the canvas(油画布) with two dollars."

Answer 2

第10题当然选can't， 因为他这幅画值20美元（Twenty dollars.), 而这个妇女只出价2美元，当然老板就说不能买了But you can't buy the canvas(油画布) with two dollars.

追问

the canvas和the picture是怎么个关系