In What Way Is the Log-Concentration Effect (Dose Response) Curve

In what way is the log-concentration effect (dose response) curve of an agonist drug modified in the presence of a competitive receptor antagonist?

A) shifted to the right without a change in slope or maximum effect
B) shifted to the left without a change in slope or maximum effect
C) shifted to the right with a decreased slope and maximum effect
D) shifted to the left with a decreased slope and maximum effect
E) not shifted right or left but shows a decreased maximum effect