Which of the following regarding dose-response curves is false?

why is the answer both a and d in this question? I thought it was only a:

Which of the following regarding dose-response curves is false?

  1. A higher ED50 is associated with greater potency
  2. Therapeutic index (TI) is calculated by dividing the TD50 by the ED50
  3. At Emax all receptors are assumed to be saturated
  4. Reversible competitive antagonists prevent a drug from reaching Emax inthe presence of excess agonist
  5. Both A and D 
 
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