题目

完形填空A first-grade student whose best friend is battling cancer thought that hours of playing and telling jokes was not enough - he wanted to show his buddy just how he understood the   1   time he was going through.Seven-year-old Vincent Butterfield   2   the only way was to cut off his hair in support of his best friend Zac who lost his hair through chemotherapy(化疗).Zac Gossage is receiving treatment but   3  goes to Union Central Elementary School in Missouri every day,   4  that's where he gets to play on the swing with Vincent. Vincent found out that Zac might  � Last night, before the exam, I went over the lessons ________ I couldn't keep my eyes open to stay awake any longer． [　　] A．before B．until C．unless D．when