Which of these songs was the first No. 1 "Billboard" hit single for the band "Chicago"?

The song "If You Leave Me Now" by the American rock group Chicago was from their 1976 album "Chicago X". It was written and sung by bass player Peter Cetera and released as a single in July 1976. It is also the title of a Chicago compilation album released by Columbia Records in 1983.

The single "If You Leave Me Now" topped the "Billboard" Hot 100 in October 1976, and stayed there for two weeks, making it the first No. 1 hit single for the group as well as hitting No. 1 on the Easy Listening charts.

"If You Leave Me Now" was also Chicago's biggest international hit song, topping the charts in other countries such as the UK, Australia, Ireland, Canada, and Netherlands. In the UK it maintained the No. 1 position for three weeks. It was also one of five "non-disco" songs in the US to make it to No. 1 within a period of time running for nine-month in the year 1976.

According to music media writer Zachary Houle of "PopMatters", "This song was so pervasive on radio (at its release) in New York it could be heard playing on four different stations simultaneously. This being a peculiar situation since each station had a different operational format from the others."