A block of mass M, supporting another block of mass m, is moved horizontally with an increasing acceleration from zero. If μ is the coefficient of static friction between the blocks, the acceleration at which the upper block slips on the lower block is
1
μm/g
2
μg
3
\(\frac{{\mu mM}}{{m\;+\;M}}\)
4
\(\frac{{\mu mM}}{{m\;-\;M}}\)
5
None of these