Drugs with Low Efficacy Bind to Receptors but Do Not

Drugs with low efficacy bind to receptors but do not fully activate them. Such "partial agonists" can act as either as a weak agonist (in the absence of a full agonist) , or as a competitive antagonist (if a full agonist is present) . Which curve best reflects the effect produced by this type of agonist when it is administered alone?

A) A
B) B
C) C
D) D
E) E