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
Percentage of offspring affected by colour blindness when a healthy male and heterozygous female marry 
Select and explain. 
A. 0, B. 25%, C. 50%, D. 100%
B: 25%
Answer: B: 25% 
Explanation: None