Use of English PRO

Stonehenge is (0) A prehistoric ring of giant standing stones situated in the middle of a vast complex of Neolithic burial grounds near (1) ............ Salisbury, England. Its construction, which is estimated to have taken more (2) ............ 1500 years to complete, is believed to have commenced as long ago (3) ............ 3000 BC. Archaeologists generally agree that the site's purpose was probably ceremonial and connected with death and burial, but, as we have no historical record of the people who built Stonehenge, there is no way to (4) ............ sure. There are many intriguing questions related to how it was built. Just (5) ............ the giant stones, each weighing several tonnes, were transported to the site from many miles away is unknown, but it is assumed that many people were involved in the process. The stones were possibly rolled across the country (6) ............ top of a track of wooden logs. On the summer solstice, which is the longest day of the year, crowds of people gather at the site to witness the sun rise above the monument, a ritual that has occurred for many years. (7) ............, visitors to Stonehenge are no longer allowed to walk amongst the stones, (8) ............ to touch them. Centuries of tourism have already caused too much irreversible damage to this unique monument.