Because it prevents them from breathing, since the ant is blocking off their airway.
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! :)
