A force F = t is applied to a block A as shown in figure, where t is time in seconds. The force is applied at t = 0 seconds when the system was at rest. Which of the following graph correctly gives the frictional force between A and horizontal surface as a function of time t. [Assume that at t = 0, tension in the string connecting the two blocks is zero].
 

 

 

Let   be the mass of blocks A and B respectively. As the force F increases
from 0 to   , the frictional force f on block A is such that f = F. When $F={\mu }_s{m}_{A}g$  the frictional force f attains maximum value $f={\mu }_s{m}_{A}g$. As F is further increased to   the block A does not move. In this duration frictional force on block A remains constant at