Which of the following options is the best substitute for the underlined word in statement I?

Directions: In this passage, errors or parts of a sentence, have been left out/have been underlined. To correct these errors and correctly complete the passage, choose the correct option.

Statement I: Long, long ago, a young man returned to his native village for holidays. Although he wanted  to enjoy nature, he was sitting under an apple tree in his mother’s beautiful garden.
Statement II: He saw an apple felled to the ground. 
Statement III: They began to wonder why the apple fell towards the ground. He concluded that 
Statement IV: this happened _________.