The Number of Molecules in a Helium (He) Sample That $l$

Multiple Choice

The number of molecules in a helium (He) sample that occupies 50 $l$ at STP is

A) 6.7 $\times$ 10¹⁸.
B) 3.4 $\times$ 10²¹.
C) 2.0 $\times$ 10²⁴.
D) 1.3 $\times$ 10²⁴.
E) 1.3 $\times$ 10¹⁸.