5.Melatonin may increase symptoms associated with autoimmune diseases , so individuals with these disorders -from taking melatonin. A)must be refraining B)may have refrained C) would refrain D) could have refrained E) ought to refrain

## Step 1<br />The problem is a multiple-choice question that tests your understanding of modal verbs in English. The modal verbs in question are 'ust', 'ay', 'would', 'could', and 'ought to'. Each of these verbs implies a different level of necessity or possibility.<br /><br />## Step 2<br />The sentence in question is "Melatonin may increase symptoms associated with autoimmune diseases, so individuals with these disorders - from taking melatonin." The modal verb 'ay' in this sentence implies a possibility, not a certainty.<br /><br />## Step 3<br />Given the context of the sentence, we need to choose the option that best fits the implication of 'ay'. The correct answer is 'ought to refrain', a strong recommendation or advice, not a strict necessity.