Someone help me please!!

Which of the following best summarizes the entire passage

Which sentence from the story is an example of conflict? “here were many men, and countless dogs, and buck found them all at work.” “buck had been purposely placed between dave and sol-leks so that he might receive instruction.” “by afternoon, perrault, who was in a hurry to be on the trail with his despatches, returned with two more dogs.” “that night buck faced the great problem of sleeping.”

Desalination is expensive because it requires