Many trees and flowers in our school last year and they made our school a beautiful garde

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A. have planted
B. are planted
(答案→)C. were planted
D. will be planted
解析：考察时态及语态。句意：去年我们学校种了许多树和花草，他们让我们学校变成了一个大花园。结合语境可知前文描述的是过去发生的动作，故用一般过去时态。句子主语是动作对象，故用被动语态。选C。