PLEASE HELP ME ASAP (REPOST) What is meant by the term bystander effect?

A. People can find that a group may be too large to have any cohesiveness.
B. Individuals tend to take more responsibility for reporting crimes than do groups of people.
C. Individuals may have more difficulty describing a person who committed a crime.
D. People tend to have more fun when they are in groups instead of being alone.

AP_EX - SOCIOLOGY