Why couldn’t they meet us at five o’clock—Because they were delayed by _________.

The explorers failed to find the cave because it was not locatedthey had expected it to be.

(2013·潍坊市高三二模)As we all know, ________ medical examination will help us find our health problems as early as possible.

You __________ football after school. Why not go home and do your homework first

The Mekong River, which________ several countries in Southeast Asia, ______to be playing a significant role in the region.