问题
选择题
That was the first time ______ I ______ face to face with the president. [ ]
A. when; had come
B. that; have come
C. when; came
D. that; had come
答案
答案：A
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That was the first time ______ I ______ face to face with the president. [ ]
A. when; had come
B. that; have come
C. when; came
D. that; had come
答案：A
PbO2+2Pb(NO3)2+2H2O。铅丹的化学式是 ，此铅丹中铅的化合价是 。由此反应可推测PbO是(填“酸”或“碱”) 性氧化物。