To prevent a repetition of this dreadful occurrence, we must discover the ______ element in the food that was served.

A．unknown

B．toxic

C．kind

D．shameful

简述网点的定义及作用。

底色去除工艺的原理和特点是什么？

光源发出的光为什么会有颜色？光源的色光受什么因素的影响？

请解释为什么从事暗室工作的人员，在进入光亮环境之前要戴上红色眼镜？

The frequent ______ from cold to warm weather this spring have caused much illness.

A．transmissions

B．transactions

C．transformations

D．transitions