题目

阅读下面短文，在空白处填入 1个适当的单词或括号内单词的正确形式。PMstands for particulate (微粒的)matter: the term for a mixture of solid particles(粒子) and liquid droplets in the air. Some particles are large or darkenough to be seen by our eyes. Others are so small that they can only (discover) with a microscope.Theseparticles come in many sizes and (shape) and can be made up of many differentchemicals. Some come directly from a source, such as fields or fires. Mostparticles form the atmosphere as a result of complexreactions of chemicals, are pollutant顺次连结等腰梯形各边中点所得的四边形是 [     ]A.矩形B.菱形C.正方形D.平行四边形