A 30-year-old man has a history of ulcerative colitis for five years.He has now had acute flare over one week，and developed high fever，bloody diarrhea with bowel movements more than 10 per day，as well as abdominal distention，cecum with a diameter of 13cm，and transverse colon diameter of 8cm on X-ray.Which of the following is the most likely diagnosis
A: Ulcerative colitis with mild attack
B: Ulcerative colitis with moderate attack
C: Ulcerative colitis with colonic perforation
D: Severe (or fulminant) ulcerative colitis with toxic megalon