Dr. Reynolds developed a treatment for social anxiety disorder. In his initial test of the treatment, he found it was effective in decreasing social anxiety symptoms in adults who only had social anxiety. Because Dr. Reynolds knows people who have social anxiety disorder also often are depressed, he designs a second study to test whether his treatment decreases social anxiety symptoms in two different groups of people - those who have social anxiety disorder only and those who have both social anxiety disorder and depression. This second study would be which of the following?
a- direct replication
b- meta-analysis replication
c- conceptual replication
d- replication-plus-extension