According to specialists, car engines pollute the air with carbon dioxide, so we (should/must/has to) try to use other means of transportation, such as bikes.
should
We (don't have to/has to/must) be careful with food waste, because it can produce greenhouse gases, and because food scarcity is very high in many countries.
must
We (mustn't/don't have to) waste water, because water availability is a really big problem.
mustn't
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