C.
Sometimes it's a physical thing with a growing kid. My brother and I had a small urethra, the "hole" by which the urine leaves the body. So we were "pushing", using muscles to urinate. This caused the bladder to be small, I seem to remember them saying there was a bunch of muscle around the bladder limiting it's size. So we both had surgery to enlarge the urethra, and soon had no problems.
Ask your pediatrition.