句子填寫...instead of...

題目：

句子填寫...instead of...
Last month they decreased their production _____ their production.
答案instead of increased
爲什麼不不是instead of increasing?

解答：

本人還是認爲「instead of increasing」是對的,of後面應該是名詞或動名詞形式,估計答案是錯的!