7. The circles $P, Q$ and $R$ are all tangent to each other. Their centres all lie on a diameter of $P$, as shown in the figure.
$$
\text { What is the value of } \frac{\text { circumference of } Q+\text { circumference of } R}{\text { circumference of } P} \text { ? }
$$

A 1
B $\frac{1}{2}$
C $\frac{1}{3}$
D $\frac{1}{4}$

E more information needed