Which of the following states a FACT about an adverb?

a. Occasionally, sometimes, often, frequently and usually can also go at the beginning or end of a sentence

b. Adverbs of indefinite frequency mainly go in MID position in the sentence. They go before the main verb (except the main verb "to be")

c. Rarely and seldom can also go at the end of a sentence (often with "very")

d. All of the above.