A:My father asked me to look after my sister at home.
B:My father asked me to take my sister back home.
C:My father asked me to go home together with my sister.
已知平面向量a→=1,2,b→=2,1,c→=x,y,且满足x≥0,y≥0。若a→·c→≥1,b→·c→≥1,z=-a→+b→·c→,则 ( )
编号
锌的质量/g
锌的形状
温度/℃
完全溶解
的时间/s
A
2
薄片
5
400
B
15
200
C
25
100
D
35
t1
E
细小颗粒
t2
F
粉末
t3
G
薄片(含少量杂质Cu)
t4