化学在生产和日常生活中有着重要的应用．下列说法正确是（　　）

A．高纯度的硅单质广泛用于制作光导纤维

B．向煤中加入适量石灰石，在煤燃烧时SO₂最终生成CaSO₄，可减少对大气的污染

C．将盛满二氧化氮气体的试管倒立在水中，可观察到溶液充满试管

D．为测定熔融氢氧化钠的导电性，可将氢氧化钠固体放在石英坩埚中加热熔化

Currently 94% of Web users use either Internet Explorer or Firefox, but recently some new (31) have been developed that are beginning to attract attention. The browser Opera is becoming very (32) because of its speed, it is currently the world’s fastest browser, and because it is much smaller (33) existing browsers (it can almost fit on a single diskette). It can also (34) the last Web page you visited, so the next time you surf, you can start where you left off. And like the big two, you can get it for free; the catch is that you have to watch blinking ads in one comer, or pay ＄40 for the ad-free (35) of Opera.

31（）

A.readers

B.machines

C.techniques

D.browsers