选C.proved
解析：
这个动词能用主动,也能用被动.但是A、B、D的谓语动词都是跟复数的主语,所以排除了.
只能选C.
成份：
主语：the fertilizer
谓语动词：proved （系动词）
表语：effective
状语：after tested in the field
the fertilizer ()effective after tested in the field
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