波峰焊焊接前，对印制板预热的目的是防止元件突受高热而损坏，同时提高了助焊剂的活化。
参考答案：对
—Tom does well in all his subjects, but he never shows off.
—I agree, He is very .
A．modest
B．clever
C．easy-going
D．generous
患者，女性，50岁，近三天来剑突偏右上腹剧烈疼痛，呈阵发性。伴畏寒、高热，今日黑便三次而来诊，无呕血。体检：皮肤巩膜可疑黄染，剑突下及右上腹有压痛，肝区有叩痛，其黑便原因最可能为
A．出血性胃炎B．胃癌出血C．胆道感染出血D．十二指肠溃疡出血E．出血性肠炎
