Correct Answer - C
`tantheta=(PR)/(0R)=(AC)/(PC) rArr (3L//2)/(L)=(AC)/(2L) rArr AC=3L`
`tanalpha=(QR)/(OR)=(BD)/(DQ) rArr BD=(L//2)/(L)xx2L=L`
`therefore AB=AD-BD=(3L+L)-L=3L`.
For `A` to `B` observ can observe `M_(1)`
Making similar diagram for lower mirror
Total visible distance `=3L+3L=6L`
Total time `=(6L)/(u)` .