You don't give much information to go on.
Most likely it is the Guy A. Why? Assuming you had your period start on March 5 (in the first week), you'd ovulate between about March 16 and 21. Sperm can survive about 5 days. You'd VERY likely be near peak fertility on March 17.
The egg only is viable about 48 hours after ovulation. So, even if you ovulated on the 21st, the egg would not be viable in early April, so it is unlikely Guy B was the father.
Due dates mean nothing, especially with the first. All dates, even an ultrasound, are an approximation. You'll find your doctor usually bases dates on your LMP.
Based on what you provide, in my opinion, it is Guy A. A DNA test can confirm paternity.
Good luck. Hope it helps.