Three elephants A, B and C are moving along a straight line with constant speed in same direction as shown in figure. Speed of A is \[5\text{ }m/s\] and speed of C is 10 m/s. Initially separation between A and B is 'd' and between B and C is also. When 'B' catches 'C' separation between A and C becomes 3d. Then the speed of B will be

• A. \[7.5\text{ }m/s\]
• B. \[15\text{ }m/s\]
• C. \[\text{20 }m/s\]            
• D. \[\text{5 }m/s\]