If 5^21 x 4^11 = 2 x 10^n what is the value of n?

A - 11
B - 21
C - 22
D - 23
E - 24

Hi Stacey, this first question on the Prep CAT really threw me for a loop. Maybe it's obvious and I'm just not seeing it yet, but how is the answer (B) 21? Much thanks,

Joe

Some overall stats: -- 26 years old, male -- US citizen, ancestry is from India -- GMAT: 700 - 42 quant(63%), 42 verbal (95%) -- Undergrad: 3.6 GPA - Well regarded technical college, but got bachelors in Int'l Business & Mandarin. Heavily active in University groups, managing 12 employees. Work...