Answer: C grating; conniving
Explanation:
A comparison Degree is used when you want to compare one thing with another thing.
To a positive degree, the two things being compared are at the same or comparable level, therefore they are used as adjectives/adverbs
At a comparative degree, one of the two things being compared has a 'more' character than the other thing, therefore an adjective/adverb + -er or more + adjective/adverb is used.