What is the median effective dose (ED50) in a Quantal dose-effect distribution curve?
a. A dose at which 25% of individuals show the specified quantal effect
b. A dose at which 75% of individuals show the specified quantal effect
c. A dose at which 100% of individuals show the specified quantal effect
d. A dose at which 10% of individuals show the specified quantal effect
e. A dose at which 90% of individuals show the specified quantal effect