第21题: [单项选择]很可能引起氟中毒的氟摄入阈值是() A. 3mg/kg B. 4mg/kg C. 5mg/kg D. 6mg/kg E. 8mg/kg 参考答案:C 答案解析:由于氟的平均致死量相差太大,Whitford提出用很可能中毒剂量(probablytoxicdosE,PTD)作为确定需立即急诊住院治疗的指征。PTD是很可能导致急性中毒、很可能引起中毒的氟摄入阈值,
第25题: [单项选择]Perhaps I should not have done so, but I changed my mind about the new job even though I was______last week. A. to be started B. to have started C. to have been starting D. start 参考答案:B 答案解析:[解析] be的过去时+不定式完成形式表示过去打算要做而实际未做的动作,因此B项为正确答案。