Choose the best word to complete these sentences.

1. No one knows how he died, but one theory is that it ______________ have been a tragic accident.
2. The potato crops may have _________________ destroyed by insects.
3. Scientists have many theories about what might _______________causing global warming.
4. When zoologists measured the spiders, they realised that they must ____________ been at least 16cm in length.
5. I should have ______________homework more often when I was at school.
6. It is not true that the Great Wall of China can _______________ seen from the moon.