Subject :IMO Class : Class 7

The points A, B, C and D lie on a straight line. The ratio of the length of AB to the length of BC is 3 : 4 and the ratio of the length of AC to the length of CD is 2 : 1. Find the ratio of the length of BC to the length of AD.

A21 : 8

B1 : 21

C8 : 21

D5 : 6

explain please