A can do a certain work in 15 days. B is 10% less efficient than A, and C is 20% more efficient than A. All the three work together for 3 days. The remaining work is completed by D alone in 9\({1 \over 2}\) days. D alone can complete the original work in:
1
18 days
2
15 days
3
25 days
4
20 days