it was ___ she was about to speak ___ when the bell rang

这是一个强调句,第一空填when而不是whom,因在这里强调的不是说话的人而是那个时间点.第二个空应该不填吧.
it was when she was about to speak when the bell rang.

应该是when,whibh吧

应该都不是，第一个空是强调后面的whom，第二个是固定用法
it was whom she was about to speak to when the bell rang