What's does crosser mean as it is used here ?

Read the sentence a student wrote. as he listened to the haunting notes of the song the author realized he had found the inspiration for his next suspense novel. the teacher reviewing this sentence would point out that since this is a complex sentence, there should be a comma after “song.” compound sentence, there should be a comma after “realized.” complex sentence, there should be a comma after “inspiration.” compound sentence, there should be a comma after “found.”