儿童得了佝偻病,是因为缺乏含什么元素的无机盐 [ ]
A.钙
B.铁
C.碘
D.锌
A
When _______ about their opinion of the effect of modern equipment on their performance of tasks, many young people gave a positive response.
A.asked
B.asking
C.being asked
D.to ask
如图所示,一条光线从左边入射,经过方框内的光学元件作用后,从右边射出。请在方框内画出所需的光学元件及光路图(包括法线)。