天然气以（）为主要成分，而且是以气体状态从地下岩石中来到地面的。天然气是（）气体。
参考答案：碳氢化合物；可燃性
Many questions have been answered by John. He must have previewed the lessons last night, _____ he? [ ]
A. needn't
B. hasn't
C. mustn't
D. didn't
蛋白酶为什么对反转录病毒生活史很重要？
needn't 