Given a prompt and four completions, select the completion that is the most plausible in continuing or answering the prompt. This task is designed to test common sense and has various categories ranging between idioms, negated statements, polysemy, subject referencing, and quantitative reasoning. Generate either Completion A, Completion B, Completion C, or Completion D.

Prompt: Tom is good at studying. He 
Completion A: spent ten days to understand a knowledge while others may only need 1 day. 
Completion B: lost his mind. 
Completion C: is very happy. 
Completion D: spent half an hours to understand a knowledge while others may need 10 days.
Completion D