题目

The man was seriously wounded, so a doctor must be ________at once. A. looked for     B. called for        C. sent for       D. asked for   答案：Csend for派人请，本句中为被动形式。“染色”馒头中添加有铬酸铅（PbCrO4）颜色为柠檬黄，会使人体致癌，已被卫生部门明文禁用。已知铬元素（Cr）化合价为+6价，则铅元素的化合价为（ ）A. +2 B. +4 C. +5 D. +7