Past Perfect Simple versus Past Simple 3

Gap-fill exercise

Fill in all the gaps, then press "Check" to check your answers. Use the "Hint" button to get a free letter if an answer is giving you trouble. You can also click on the "[?]" button to get a clue. Note that you will lose points if you ask for hints or clues!
Fill in the correct form of the verb. Choose between the Past Perfect Simple and the Past Simple.

One day in 1940, at Lascaux in south-west France, a teenage French boy, who was out for a walk with three friends, carefully (to go) down a hole in the woods. His dog (to fall) down the hole and he (to want) to rescue it.
After a long slide, he (to reach) the dog - and a large cave. But as he only (to have) a box of matches with him, he (not/ to realize) that he (to find) one of the most remarkable caves in Europe.
Next day, however, the four friends (to return) with a lamp, and they (to be) excited to see large animal paintings on the walls and roof of the cave. They (to see) horses, bison and other animals which (once/to wander) wild around this part of Europe. But they still (not/ to know) that they (to discover) pictures that prehistoric men (to paint) more than 16,000 years before.
Other people, however, soon (to realize) the importance of the discovery, and when the news (to spread) , the public (to hurry) to see the paintings for themselves. And they did so in increasing numbers until, on 20 April 1963, the cave (to be) closed.
Unfortunately, in only twenty-three years, the dampness from the breath of so many visitors (seriously/ to damage) the paintings which (to survive) in secret for 16,000 years