The answer is A because the rest would incomplete sentences
Sternly, maybe could just be grim ( formiadable, harsh, uninviting, depressing, lack of humor, etc)
It would be systematic because your not persuading them to do anything and assertive is about your personality. Systematic means to do something according to a plan or system.
An example of a simile was " Arithmetic is where numbers fly like pigeons in and out of your head"
