Put the following four numbers in order from least to greatest: $A.\ 4 \cdot \sqrt{5}\\ B.\ 6 \cdot \sqrt{2}\\ C.\ 5 \cdot \sqrt{3}\\ D.\ 9$

Write your answer as a list of letters, separated by commas. For example, if you believe A is the smallest, B is the next-smallest, C is the next-smallest, and D is the largest, you would write A, B, C, D.