Exercise: They were warned never ... with the members of any extremist

[Duc]

Hello everyone ,
Below is an exercise that makes me very confused:
Choose the correct answer:
1.They were warned never ............. with the members of any extremist group.
A.to associate B.to assimilate C.to assign D.unreasonable
The answer in my book is D but I wonder Unreasonable is an adjective so we can't put it directly before ''warned never..."
Please explain it for me.I'm sure it is a mistake but ....
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Don't see your shade think you are great

[Torsten]

I guess it's a typo in your book. The correct answer is A.
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Test Of English for International Communication
TOEIC Preparation & TOEIC Vocabulary

[Yankee]

.
What's the name of your book, duc? Sorry if you've already told me this once before.
In all honesty, though, I think you would be better off with a higher quality book.
.

[Duc]

Yes, thank you!
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Don't see your shade think you are great