Block A is placed on block B, whose mass is greater than that of A. There is friction between the blocks, while the ground is smooth. A horizontal force P, increasing linearly with time, begins to act on A. The accelerations `a_(1)` and `a_2` of A and B respectively are plotted against time (t). The correct graph is

Explanation

The two blocks will move together with the same acceleration as long as the force of friction between them is less than the limiting friction, as the only force on the lower block B is the force of friction. Once limiting friction is reached, the acceleration of B becomes constant `=F_("lim")/("mass of B")` and the acceleration of A continues to increases at a faster rate.