If a positive zero error exists in a vernier callipers, it means that the instrument shows a reading greater than zero even when the jaws are closed and there is no object being measured. In such a case, when actual measurements are taken, the readings will be more than the actual values.
The zero error in Vernier callipers can indeed occur due to manufacturing defects or rough handling. Manufacturing defects may lead to misalignments or inaccuracies in the instrument, while rough handling can result in damage that affects the calibration of the callipers.
Hence, both (A) and (R) are correct, but (R) is not the correct explanation of (A)