I am looking after little Tom today. He's been in my house _ 8:00 this morning.
A:at
B:since
C:for
D:till
将要上钩的鱼,会被岸上的脚步声吓跑,表明 ____和 ____能够传声.
42÷____=____
、如图所示的军事观察孔,图中的虚线表示空气孔的观察范围,现在在孔中嵌入玻璃砖后观察的范围将()