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