题目内容

他的发明很偶然。It was invented ____.

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1000年前，人类在尝试制造各种药物的过程中，More than 1,000 years ago people experimenting in a ____ made the discovery,

偶然发现将硫磺，硝石，木炭，以一定的比例混合，便生产出火药。They found that if sulphur, charcoal and potassium nitrate were, with a certain the mixture would explode.

火药使军事武器从冷兵器时代开始向火器发展。Gunpowder changed the style of wars. The ____ ended and gunpowder-based weapons are now widely used.

600多年前，中国便有了原始火炮。More than 600 years ago ____ were used in China.