This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
Constant PR inverval is seen in -a) First degree blockb) Second degree - Mobiz type I block.c) Second degree - Mobiz type II blockd) Third degree block 
Select and explain. 
A. a, B. bc, C. ac, D. b
C: ac
Answer: C: ac 
Explanation: None