A First-Order Reaction Has a Half-Life of 20

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 is correct.