A．He hasn’t had time to find another apartment.
B．His mother cleaned his apartment every day.
C．He hasn’t cleaned the apartment since his mother came to see him.
D．His mother was the last person to come over to his apartment.

A．No one knows exactly how silk was made.
B．No one knows exactly when silk was made.
C．The history of the silk is a fascinating story.
D．The Chinese people kept the secret of how to make silk for as long as 4,000 years.