Which of the following types of study support external validity? a. A study using non-randomly selected participants b. A study that adds a new independent variable c. A study that is a direct...

 Which of the following types of study support external validity?

a.  A study using non-randomly selected participants

b.  A study that adds a new independent variable

c.  A study that is a direct replication

d.  A failed replication study

B is the answer but why adding new independent variable can support external validity? (please type, no hand-writing)