One of my grammar books 日本語総まとめN2 文法 lists both of the grammar forms 「きっかけに」 and 「契機に」 in the same definition.
However, in the example question a choice is given between them:
その事件（a. の契機に b. をきっかけに)、商品の検査体制が見直された。
The book lists "b" as the correct answer, yet I'm still puzzled as to why either of them would be more appropriate than the other.
Is it the case that 「契機に」 has a much more limited/specific use compared to the other?