Questions about example sentences with, and the definition and usage of "Ecologist"

A: × Some ecologists have been warning us that electric cars are assumed to be good for the environment, but we might consume more fossil fuel to manufacture them and generate the electricity to drive them.
✓ Some ecologists have claimed that electric cars are good for the environment, but in reality, it costs a lot of fossil fuel to manufacture them and generate the electricity to drive them.

A: × The ecologist has (warned/cautioned/alerted) us that the petroleum is not only a blessing but also a curse.
✓ The ecologist (warned or cautioned) us that oil is not only a blessing, but also a curse.

A: I believe that D and E are correct grammar, because it makes sense to say “I have traveled here by horseback” as well as “I have traveled here on horseback”. Because a bicycle is a piece of machinery, you would say “by bicycle” just as you would if you were saying “I have traveled here by car” or “I have traveled here by train” or any other means of transportation of that sort.

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