sorry, im really dumb.. but here's the question

A person researching business-to-business communications found 90% of people used email as their primary means of communication. If there were 20 B2B operators contacted for a future research study focused solely on email communication, what is the probability that none of the 20 will use email in B2B as their primary form of communication?

i know the answer is a

but can someone just explain to me, why didnt they sub the values in the binomial formula? coz i thought that you have to use the full formula when calculating the binomial, but then why did they only use a part of it?