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In the given figure, $QR$ is a common tangent to the two given circles touching externally at $A$. The tangent at $A$ meets $QR$ at $P$. If $AP = 4.2$ cm, then the length of $QR$ is :
- (a)$4.2$ cm
- (b)$2.1$ cm
- (c)$8.4$ cm
- (d)$6.3$ cm
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(C) $8.4$ cm