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
Which of the following allows binding of HIV to CD 4 cells  - 
Select and explain. 
A. gp 120, B. gp 41, C. Proteins formed by expression of gag gene, D. ccR5
A: gp 120
Answer: A: gp 120 
Explanation: None