Even$-$numbered Fresnel zones:

$($a$)$ Will cause reflected signals to arrive at the distant end receiver out of phase with the direct radio beam. Thus, their impact will detract from $($weaken$)$ the total, composite signal arriving at the receiver.

$($b$)$ Will cause reflected signals to arrive at the distant end receiver in phase with the direct radio beam. Thus, their impact will be to supplement the total, composite signal arriving at the receiver.

$($c$)$ Will have no effect on the composite signal at the distant end receiver

$($d$)$ Will only affect the composite signal at the receiver on rainy days