Luckily, the back tires of their car stayed on the road. Otherwise, the young couple would

have driven right into a pit twenty feet wide and thirty feet deep! The man and woman were coming home from a party. They were enjoying the landscape around Swansea, Wales. Suddenly, they found the front of their car leaning into a huge hole. The car barely hung onto the edge of the pit. It swayed back and forth like the arm of a balance. In their precarious position, the couple knew that each movement they made could be a matter of life and death. Slowly, slowly, they edged toward the backseat. Then each opened a back door. And on the count of three, they jumped out together. The accident was so scary that they ran a long way before they calmed down. But later they returned to see what had happened. They found that a big chunk of the road had sunk into the ground! And at the bottom of the pit lay their car—roof down and wheels up. Was this mystery of the sunken road ever solved？ It turned out that an abandoned mine shaft lay under the road. It had collapsed and taken the pavement with it. Layers of tunnels intersect beneath the city of Swansea. The tunnels were built so many years ago that no one knows where they end or begin. The tunnels are shaky, like those that ants build in the sand. No one knows when the entire city might collapse.

This passage is primarily about______.

A．driving carefully

B．the city of Swansea, Wales

C．a road that caved in

D．tunnels that ants build