Dexter and Prexter are competing with each other in a friendly community competition in a pool of 50m length and the race is for 1000m. Dexter crosses 50m in 2 min and Prexter in 3 min 15 sec. Each time they meet cross each other, they do handshake's. How many such handshake's will happen if they start from the same end at the same time?

• A. 19
• B. 18
• C. 20
• D. 17