People who live in hot, dry climates often rely on
for their drinking water

Some possible answers could be desalinization(removing salt from water), an oasis, and underground water sources(like ground water which can remain underground for thousands of years)

i believe it's c. the trait for large beaks will be passed on to future generations through natural selection.

since eventually smaller beaks would have to find another source of food or even eventually just die out from the lack of food.