$v_A=10 \,ms ^{-1}-10 t$
$v_B=0-10 t$
$v_{A B}=v_A-v_B=10-(10 t)-(-10 t)=10-10 t+10 t=0$
$\Rightarrow v_{A B}=10 \,ms ^{-1}$
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$A.$ When small temperature difference between a liquid and its surrounding is doubled the rate of loss of heat of the liquid becomes twice.
$B.$ Two bodies $P$ and $Q$ having equal surface areas are maintained at temperature $10^{\circ}\,C$ and $20^{\circ}\,C$. The thermal radiation emitted in a given time by $P$ and $Q$ are in the ratio $1: 1.15$
$C.$ A carnot Engine working between $100\,K$ and $400\,K$ has an efficiency of $75 \%$
$D.$ When small temperature difference between a liquid and its surrounding is quadrupled, the rate of loss of heat of the liquid becomes twice.
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