Two blocks of masses $m$ and $M,(M>m)$, are placed on a frictionless table as shown in figure. A massless spring with spring constant k is attached with the lower block. If the system is slightly displaced and released, then ( $\mu=$ coefficient of friction between the two blocks) A. The time period of small oscillation of the two blocks is $T=2 \pi \sqrt{\frac{(m+M)}{k}}$ B. The acceleration of the blocks is $a=-\frac{k x}{M+m}$($x=$ displacement of the blocks from the mean position) C. The magnitude of the frictional force on the upper block is $\frac{m \mu|x|}{M+m}$ D. The maximum amplitude of the upper block, if it does not slip, is $\frac{\mu(M+m) g}{k}$ E. Maximum frictional force can be $\mu(\mathrm{M}+\mathrm{m}) \mathrm{g}$. Choose the correct answer from the options given below :