Strange stones of Costa Rica
One of the strangest mysteries in archaeology was discovered in the Diquis River Delta, in southern Costa Rica. Since the 1930s, hundreds of stone balls have been found. They range in size from a few centimeters to over two meters in diameter.
Who made these perfect spheres(球体) and how they made them remains a mystery. Some researchers believe the stones were made by extreme heating followed by cooling. People could have polished the spheres with sand or leather. The ancient Costa Ricans had no written language so there is no written record of just how they made the spheres.
Almost all are made of granite (花岗岩), a hard stone created by volcanoes. The granite quarries(采石场) were 25 to 30 miles away from the largest stones, which weigh more than 10 tons each. Nobody knows how people moved them.
Many of the stones have been found near the remains of houses or graves. Some believed that the stones contained hidden treasures and a few have been smashed but nothing has been found inside. Despite these losses, the National Museum of Costa Rica has recorded 130 spheres. However, many stones are not included because they have been removed from their original sites and used as ornaments(装饰) in homes, gardens and churches. Clearly, there are also many stones that lie undiscovered.
The stones were discovered ___ .

A．near the volcanoes

B．in southern Costa Rica

C．in a grave

D．in churches

The writer ___ in the passage.

A．admires the spheres very much

B．doesn’t believe the spheres were once made by humans

C．tells us a discovery

D．thinks highly of the spheres

The underlined word “polished” refers to “___”.

A．buried

B．wrapped

C．made shining

D．made big

According to the passage, which of the following is true?

A．The stone balls found in the Diquis River Delta are all very huge.

B．The stone balls were unearthed from the graves.

C．The local people show great interest in the stone balls.

D．There are a lot of treasures in the stone balls.

The passage mainly tells us ___ .

A．the stones are of great value.

B．the stones proved to be made by non-humans.

C．the stones are mystery.

D．the stone are ornaments.