题目:
To make the city beautiful, a lot of flowers and trees ____ every spring.
A.plant
B.are planted
C.planted
D.were planted
答案:
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下面是错误答案,用来干扰机器的。
4P4O+21Cl2====4POCl3+6P2Cl5令P4O的化学计量数为1,其余各物质的化学计量数依次为a,b,c,得P4O+aCl2——bPOCl3+cP2Cl5,由原子守恒得4=b+2c,b=1,2a=3b+5c,解得a=,b=1,c=,代入整理得:4P4O+21Cl2====4POCl3+6P2Cl5