In figure-6.9, a microwave transmitter a height a above the water level of a wide lake transmits microwaves of wavelength `lambda` towards a receiver on the opposite shore, a distance x above the water level. The microwaves reflecting from the water interface with the microwaves arriving directly from the transmitter. Assuming that the lake width D is much greater than a and x, that `lambda gtgt a`, at what values of x is the signal at the receiver maximum?