The answer is D.
"Keep", "factor", and "calculate" are all verbs in the list of things the new system allows teachers to do. The other sentences are inconsistent with their list of verbs/adjectives/nouns. Thus, option D is the only one that is grammatically correct.
I am not too good at explaining, so let me know if you are still confused.
Hope this helps! :)
Either criticize or condemn. If neither of these are options, let me know.
D. The conversation you write must sound natural for the characters.
This answer is correct because conversation between characters can add humor. However, if the dialogue does not match the characters' personalities it will seem contrived and unnatural. This will lead to the characters being unreliable and take away from the overall writing. Some of the other statements could be considered correct as well in certain situations; however, they are not correct in all situations where as D is correct in all.
