Answer:
C. A blimp is lighter than-air.
Step-by-step explanation:
This sentence is the example of comparative degree in grammar. There is three major degrees in the degree of comparison such as (i) Positive degree, (ii) Comparative degree and (iii) Superlative degree. Blimp is small airship or barrage balloon which is floating on air because of its weight compared to the surrounding air atmosphere. In this sentence, blimp is compared with the air. This sentence compares two things and it comes under the comparative degree. If you are using anything as a compound to modify a noun, it would be hyphenated.