题目

It was her giggling that drew my attention.  Note taking really wasn't all that funny. Walking over to the offender, I asked for the __16__. Frozen, she refused to give it to me. I waited, all attention in the classroom on the quiet __17__ between teacher and student. When she finally__18__ it over she whispered, “Okay, but I didn't draw it.” It was a hand－drawn__19__ of me, teeth blackened and the words “I'm stupid” coming out of my mouth. I managed to fold it up calmly. My mind, __20___, was working angrily as I struggled not to __21__ . I figured I knew the two most like某种药物可以阻断蟾蜍屈肌反射活动。下图为该反射弧的模式图。A、B为神经纤维上的实验位点，C为突触间隙。下列实验结果中，能够证明这种药物“在神经系统中仅对神经细胞间的兴奋传递有阻断作用”的是(　　) ①将药物放在A，刺激B，肌肉收缩　②将药物放在B，刺激A，肌肉收缩　③将药物放在C，刺激B，肌肉不收缩　④将药物放在C，刺激A，肌肉收缩 A．①③　　　 B．②③ C．①④　　　 D．②④