6. Two swimmers start swimming simultaneously from opposite ends of a 90-meter-long swimming pool, one at a speed of 3 meters per second and the other at a speed of 2 meters per second. They swim back and forth for 12 minutes. If the time taken to turn around is negligible, then the number of times they pass each other is ( ).
(A) 19
(B) 20
(C) 21
(D) 24
6. B.

The swimming conditions of the two swimmers within 3 minutes are shown in the graph below:
After 3 minutes, both return to the starting point as they were at the beginning. Within 3 minutes, they pass each other 5 times (as can be seen from the graph, i.e., the intersections of the dashed and solid lines, including the 90th second). In 12 minutes, there are a total of $12 \div 3 \times 5=$ 20 (times).