Rectangular block B, having height `h` and width `d` has been placed on another block A as

Rectangular block B, having height `h` and width `d` has been placed on another block A as shown in the figure. Both blocks have equal mass and there is no friction between A and the horizontal surface. A horizontal time dependent force `F = kt` is applied on the block A. At what time will block B topple? Assume that friction between the two blocks is large enough to prevent B from slipping.