Which of the following statement$\left(\mathrm{s}\right)$ is/are correct?

The tension in the string connecting the blocks $A$ and $B$ kept on smooth surface is

The tension in the string $AB$ is (pulley and string are massless).  Take $g=10 \mathrm{m} {\mathrm{s}}^{-2}$

If the mass of the rope ${\mathrm{OO}}^{\text{'}}$ is $M,$ then tension at mid-point $A$ is

A $3 \mathrm{kg}$ block is resting on a truck moving with acceleration $2 \mathrm{m} {\mathrm{s}}^{-2}$ on a horizontal road. Force of friction on the block is

A body is moving on a circular track of radius $R$ at constant speed as shown in figure. If normal reaction at $C$ is $N,$ then

Tension in string $BC$ is (if string and pulley are massless)