Answer:
for; by.
Explanation:
A preposition can be defined as a word that shows or illustrates the relationship between a pronoun or noun and other words in a sentence. Some examples of a preposition used in various literary works in English language are up, below, after, by, against, for, over, at, to, etc.
The main purpose of a preposition as a part of speech is to introduce an object (of, upon), indicate a timeframe (from, by, over), show direction (to, across, along), location or place (at, up, after, below) and to illustrate the spatial or sequential relationship between two or more things, people, place, etc.
Therefore, the most appropriate or suitable preposition to complete the sentence are for and by. Now, we have the complete sentence as;
"When a person visits a private investigator for the first time, he or she is asked a number of questions by his/her assistant."
