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: Dental
Enamel knot expresses the growth factor, which can be: 
Select and explain. 
A. Slit 1, B. FGF 4,9, C. BMP 2,4,7, D. All of the above.
D: All of the above.
Answer: D: All of the above. 
Explanation: None