1. malaria 2. rivers had terrible waterfalls..too dangerous...until the steam engine was invented it was incredibly dangerous 3. deserts....they didn't know how to travel through the deserts and find an oasis which made it very difficult to travel through
They allowed for much stronger and more durable weapons.
Like, Same, Similar... if your have trouble with synonyms or antonyms look on Thesaurus.com .... they'll give you bunches of answers..
