选D，on doing介词短语表示一.. 就，相当于as soon as，As soon as he stepped out of the taxi， he was seized by the police. 常见的on arriving/.hearing.表一.. 就的连词还有the minute/moment/second/instant等。
答案是D. On his stepping, 意思是“在他走出出租车的时候”
整句话的意思是：他一走出出租车就被警察抓住了。
on有“在......之时”的意思
答案选D，
On his stepping out of the taxi, he was seized by the police.
本句的要点在于on+n./doing形式,表示“一...就...”
不会就选c3长一短选最短拿双day这么简单的道理喽。
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