It was a mistake for the test writer to choose an unlikely time, but do not let logic get in your way when taking grammar tests; choose the best grammatical answer. C is not possible because it is unnaturally redundant: 'pm' = 'afternoon'. The only other reasonable answer is B.
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Lcchang Thank you for your advice. However, as a non-native speaker learning English at school, we are always asked to put the small time before the big one. Therefore, I think answer B may not be proper because Saturday is longer than afternoon. Why I choose C is that I have