Which of the following infinitives functions as an if-clause？A．To be a good citizen, one m

Which of the following infinitives functions as an if-clause？

A．To be a good citizen, one must obey the law.

B．He opened the door to find the room empty.

C．I came here to study English grammar.

D．To hear you sing, people might take you for a girl.

请帮忙给出正确答案和分析，谢谢！