若焦距为4的双曲线的两条渐近线互相垂直，则此双曲线的实轴长为（    ）

鸦片战争后，中国自给自足的自然经济逐步解体，这种“解体”的含义是（    ）

    A proverb says, “Time is money”. But in my opinion, time is even ____precious than money. Why? Because when money____(be) spent, we can earn ____back. However, when time____(go), it will never return.

    It is known to all that the time we can use is ____(limit). Therefore, we should make use____our time more ____(full) to study hard so as to serve our country in____future. But it is a pity that there are a lot of persons____do not realize the value of time. They spend their precious time smoking, drinking and playing.

    In a word, we should form a good habit of ____(save) time. Do not put off what can be done today till tomorrow.

化学使世界变得更加绚丽多彩．如图是物质间发生化学反应的颜色变化，其中无色溶液X是（　　）