A small block is loaded onto the storage platform of a lorry, at a distance d away from its open edge, as shown in the figure. The static and kinetic coefficients of friction are μs and μk respectively. At time t = 0, the lorry accelerates from rest towards the right with a magnitude of a = Bt, where B is a constant. The block grips onto the platform until it starts slipping at t = t1, and eventually falls off the lorry at t = t2. The value of d = .