一船自西向东匀速航行上午10时到达一座灯塔P的南偏西75°距塔68海里的M处,下午2时到达这座灯塔的东南方向的N处,则这只船航行的速度为( )
A:1762海里/小时
B:346海里/小时
C:1722海里/小时
D:342海里/小时
下列生物体内肯定不存在A-T碱基对的是( )
例 ,计算689-98=____689-100=____689-102=____
He turned from side to side, eyeing himself in the mirror with a serious, studied ______ that soon changed into a smile.