无内胎轮胎的缺点是（）和自粘层容易漏气。
参考答案：密封层
甲肝病毒是( )
A．杯状病毒B．嗜肝DNA病毒C．缺陷病毒D．黄病毒E．微小RNA病毒
The young men walked _____ the forest and came to a big river at last. [ ]
A. on
B. over
C. through
D. across
