$(A)$ $\alpha+p=2 \beta$
$(B)$ $p+q-r=\beta$
$(C)$ $p-q+r=\alpha$
$(D)$ $p+q+r=\beta$
$(A)$ Error $\Delta T$ in measuring $T$, the time period, is $0.05$ seconds
$(B)$ Error $\Delta \mathrm{T}$ in measuring $\mathrm{T}$, the time period, is $1$ second
$(C)$ Percentage error in the determination of $g$ is $5 \%$
$(D)$ Percentage error in the determination of $g$ is $2.5 \%$
$Reason$ : Relative velocity of $P$ w.r.t. $Q$ is the ratio of velocity of $P$ and that of $Q$.