C I'm 99% positive it is the answer (curse this 20 characters or less.)
Earth day has been celebrated around the world each spring since it began in 1970
Modal helping verbs can be used to indicate a mood or tone of a verb in a sentence.
A modal assisting verb affects the main verb in this sense by expressing necessity or possibility. The modal verbs include can, could, may, and might. Modal verbs, often referred to as modal auxiliaries, are used to express the concepts of capability, likelihood, necessity, permission, and duty. These verbs never change their form.
An auxiliary verb known as a modal verb is used to indicate modalities, which are the states or "modes" in which a thing can exist. Examples of modalities are a possibility, ability, prohibition, and necessity. The modal verbs should, must, will, might, and could are a few typical examples.
Modal verbs are most usually employed in academic writing to denote logical possibility and least frequently used to denote permission. For each of the eight tasks that modal verbs can serve in academic writing, they are enumerated and ranked from strongest to weakest.
Learn more about modal verbs here:
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I looked it up and it said immediately was adverb