(A) is a gorgeous beach, which stretches on for a mile, and a waterfall that provides fresh water for the many people - Subject Verb Agreement error.
(B) is a gorgeous beach, which stretches on for a mile, beside a waterfall that provides fresh water for the many - Correct
(C) there is a gorgeous beach stretching on for a mile and a waterfall that provides fresh water for many of the people - Same as A
(D) there is a gorgeous beach stretching on for a mile and a waterfall providing fresh water
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(B) is a gorgeous beach, which stretches on for a mile, beside a waterfall that provides fresh water for the many - Correct
(C) there is a gorgeous beach stretching on for a mile and a waterfall that provides fresh water for many of the people - Same as A
(D) there is a gorgeous beach stretching on for a mile and a waterfall providing fresh water
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