What is the product of 4.2 ×107 and 5 ×108, written in scientific notation?
9.2 ×107
2.1 ×108
9.2 ×108
2.1 ×1015
2.1 × 1016
4.2 ×5× l07 × 108= 21×1015=2.1 × 1016，故本题选择E项。