The correct answer would be a. Radio talk shows are usually more casual in hopes of being more appealing to listeners. Also, process of elimination would work well here because no station would ever use a casual tone of voice on horrific events such a natural disaster or war.
The adverb clause in this sentence is "so that we can go to the basketball game". It explains the main verb "write" more thoroughly, and gives us an explanation "why" they have to do finish their composition.
B! The answer needs to be clear and straight forward
