The thief ran away at the _____ of a policeman.
A.sight
B.screen
C.sound
D.seeing
答案:A
小明坐在行驶的汽车中,以______为参照物小明是运动的,以______为参照物小明是静止.
水杨酸分光光度法测定水中铵时,如何测定次氯酸钠溶液中游离碱(以NaOH计)?