The sketch below shows a radio antenna located at the edge of a calm lake, which has a receiver Rat the center of the dish at a height a/2 above the ground. This is picking up a signal from a distant radio-emitting star which is just rising above the horizon. However, the receiver also picks up a reflected signal from the surface of the lake, which, at the relevant radio-wavelength, may be taken to be aplane. If the instantaneous angle of the star above the horizon is denoted a, the receiver R will detect the first interference maximum when = () (2) a 2a (a) arcsin (b) arcsin a (c) arcsin- (d) arsin 2 2a