A Wheatstone's bridge is balanced with a resistance of 625Ω in the third arm, where P,Q and S are in the 1st, 2nd and 4th arm respectively. If P and Q are interchanged, the resistance in the third arm has to be increased by 51Ω to secure balance. The unknown resistance in the fourth arm is
(a) 625Ω
(b) 650Ω
(c) 676Ω
(d) 600Ω