D. Neither he nor his friends studies every day of the week
The claim in the passage, "Beyond Thirst: The Global Water Crisis" is this: "Left unchecked, the crisis will only worsen."
<h3>What is a Claim?</h3>
A claim is an opinion that is usually made at the introductory part of a text. In the second paragraph of the cited passage, the author claimed that there is an existing challenge with water, which left unchecked, will only worsen.
In the body of the passage, he supported this claim with proof.
Learn more about claims here:
brainly.com/question/2748145
The narrator meets the Medical Man at the Linnaean Society in the city. The Medical Man thinks the Time Machine thing was a trick, but he can't figure out how the Time Traveller did it. The narrator goes back the following week for the Time Traveller's weekly dinner party.