The correct answer is (c) 106
To explain I would say: The preamplifier used by Goodman (1969(a)) has a CMRR of 106 dB (200,000 : 1) with a common mode input impedance of 150 MW. The preamplifier gain is of the order of 1000. The preamplifier also must incorporate the facility for ‘probe balance’ by which signals in phase with the magnet current can be selected to balance background voltages in phase with flow voltages.