Two blocks of masses 2\,kg (upper) and 3\,kg (lower) are in contact on a smooth horizontal table. The coefficient of static friction between the blocks is 0.40. A horizontal force of 10\,N is applied to the upper block. Assuming the blocks move together, the magnitude of the frictional force acting between them is
a) 7.8N
b) 6N
c) 4N
d) 0N