单选题

With second thoughts，I have decided to postpone my visit to Japan.

A In

B On

C For

D Against

解析：
句意：进一步考虑后，我决定推迟对日本的访问。这里用的是second thoughts（再思），四个选项中能与其搭配构成词组的只有on了。with，in，for与against都不可放在second thoughts之前构成介词短语。用序数词与thought构成词组，常见的只有at first thought和on second thoughts。at first thought=after first consideration，乍一想。on second thoughts=after further consideration，进一步考虑后。在此at和on均用来表示时间，符合题意。介词in与thought搭配时，只见于（to be lost／deep）in thought，且中间不加序数词，thought用单数。介词for与thought搭配时，常见的是（to be full of／to take／to have，etc.）thought for（sb. ）这一搭配。