Please explain.

22. 1 mL of E. coli culture was diluted to 100 mL and 0.5 mL of the diluted culture was plated onto an agar plate. After 12 h of incubation, 200 colonies were observed. What was the number of bacteria per mL in the original culture? (a) 2 x 10 (b) 4 x 10 (d) 2 x 10 (c) 1x 105