Why cheddar cheese should inhibit tooth decay is unclear, but Dr. Bowen, who reports ...
A. Why cheddar cheese should inhibit tooth decay is unclear
C. How cheddar cheese should inhibit tooth decay is not clear
The distinction between A and C is subtle.
GMAT experts with a 'good ear' may say instinctively that A is right and C is wrong, but even they will probably find it hard explain why exactly this is so (because they are GMAT experts, not expert copy-editors).
Anyone who is at below 40+ in verbal will not even see such a question, unless it appears as a test question. So it may not be a good use of your time to struggle with the A vs C issue.
When in doubt, if something like this were to appear on the real test, use what you know. Look for other reasons to eliminate an answer.
Notice that A uses 'UNCLEAR', whereas C uses 'NOT CLEAR'. 'NOT CLEAR' is not wrong, but 'UNCLEAR' is probably better.
Choose A, and move on.
Back to A vs C:
A says "Why cheddar should inhibit tooth decay is unclear" : no error
The following would have been fine too. They are grammatical and have much the same meaning.
- Why cheddar inhibits tooth decay is unclear
- How cheddar inhibits tooth decay is unclear
"How X should do Y is unclear" may be acceptable in a different context.
For example, I think it is acceptable to say "How we should prepare is unclear". Meaning, we know have to prepare but are not sure how to go about it.
That meaning will not fit in this question.
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