A first-order reaction has a half-life of 20.0 minutes. Starting with 1.00 × 1020 molecules of reactant at time t = 0, how many molecules remain unreacted after 100.0 minutes?

A first-order reaction has a half-life of 20.0 minutes. Starting with 1.00 × 1020 molecules of reactant at time t = 0, how many molecules remain unreacted after 100.0 minutes?
A) 1.00 × 104 molecules
B) 2.00 × 1019 molecules
C) 3.20 × 1016 molecules
D) 5.00 × 1020 molecules
E) None of these choices are correct.