Which of the following options should replace the underlined word in line 3?

Directions: This passage contains errors and omissions. Choose the correct option from the list given below to correct the corresponding error and complete the passage.

1. Once Newton invited some of his friends to his house for dinner.

2. Because they were eating the food, Newton had to go out.

3. At that time, he has remembered a problem.

4. He forgot why he had come out.

5. They went into his study and began to solve the problem.

6. When Newton did not come back for a long time, _____________________