Collins Grammar Unit 13

When fossil fuels were discovered, people _____ ______ _____ (know) the negative effects they would eventually cause as no research had been done on them.

In early civilisations, people _____ ____ ______ (hunt) for their own meat.

People _____ _____ _______ (take) more care of endangered species. Over a hundred species have become extinct in the last 50 years and it's a terrible shame.

It _____ _____ _____ (be) difficult to survive in the days before modern medicine and hygiene standards.

A new lizard has been discovered in the rainforests of Sumatra. Some experts say they _____ _____ _____ the deeper rainforests for up to a million years.

People _____ ____ (travel) across borders quite easily in the past. Nowadays, there are many more immigration rules.

Before organised forms of rule such as kingdoms or governments, people _____ _____ ____ _____ (pay) taxes.

Resentment among the citizens _____ _____ _____ _________ (develop) for a long time before the overthrow of the Empire, because popular support for a revolt had been growing for decades.