A:2NaOH+H2SO4=Na2SO4+2H2O
B:Ba(OH)2+2HCl=BaCl2+2H2O
C:Cu(OH)2+2HNO3=Cu(NO3)2+2H2O
D:KOH+HCl=KCl+H2O
10只鸽子飞回4个鸽笼,至少有一个鸽笼要飞进____只鸽子.