A conducting circular loop is placed in a uniform magnetic field of induction \( B \) tesla with...

A conducting circular loop is placed in a uniform magnetic field of induction \( B \) tesla with its plane normal to the field. Now, radius of the loop starts shrinking at the rate \( (d r / d t) \). Then the induced e.m.f at the instant when the radius is \( r \), is
(A) \( \pi r B\left(\frac{d r}{d t}\right) \)
(B) \( \pi r^{2} B\left(\frac{d r}{d t}\right) \)
(C) \( 2 \pi r B\left(\frac{d r}{d t}\right) \)
(D) \( \frac{\pi B r^{2}}{2}\left(\frac{d r}{d t}\right) \)