james ruse 2008 year 10 yearly paper (1 Viewer)

[ShilohHuxley]

can anyone explain Q.28 from james ruse 2008 year 10 yearly paper?
https://thsconline.github.io/s/?view=5108&id=0ByEFYhkkDQBKVmhUVWZXelFBMUU&n=James Ruse 2008 w. sol

 

[worldno17]

AB^2 = BD * BC (square of intercept on tangent equals product of intercepts on secant)
AB^2 = 7 * 2
AB^2 = 14

Therefore, AB = sqroot 14 (length > 0).
The answer's B.