选a啊,这是英语中的一个句型,be about to do .when引导的从句,意思是正要…这时…那应该是B啊第一空that是强调句第二空是be about to do .......whenwhen 引导的定语从句修饰前面的先行词，at five in the afternoon when I was about to get home 这部分是被强调的。A strange sound caught my attention at five in the afternoon when I was about to get home.这是正常语序