单选题 The old man returned to the house to ＿＿＿＿＿＿ all the doors were locked. 的答案

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题目内容：
The old man returned to the house to ＿＿＿＿＿＿ all the doors were locked.
题目选项
insure against
ensure of
assure of
make sure

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