A single grain of sand occupies a volume of about 0.004 cubic millimeters. Based on your previous answer, how many grains of sand are on Earth's beaches? (Express your answer to the correct number of significant figures.)
a. 2.1 x 10²²
b. 3.7 x 10¹⁹
c. 5.6 x 10²³
d. 1.8 x 10²⁵