Given the task definition and input, reply with output. In this task, you're given the title of a five-sentence story, the first four sentences, and two options for the fifth sentence as a and b. Your job is to pick the sentence option that seamlessly connects with the rest of the story, indicating your choice as 'a' or 'b'. If both sentences are plausible, pick the one that makes more sense.

Title: Graveyard. Sentence 1: Mary and Shelley walked through the local graveyard. Sentence 2: It was quiet aside from some birds and the wind. Sentence 3: Suddenly, a hand grabbed Mary's shoulder from behind. Sentence 4: Mary screamed. Choices: a. Shelley laughed as her prank succeeded. b. Gibson threw the fish back into the water.
a