$(5)$Even if task and personnel are perfectly matched, however, the ways in which group members interact may reduce the group's efficiency. $(6)$For example, high$-$status individuals tend to exert more influence in groups, so if they do not possess the best problem$-$solving skills, group decisions may suffer. $(7)$Another factor affecting group interaction and effectiveness is group size. $(8)$The larger the group, the more likely it is to include someone who has the skills needed to solve a difficult problem. $(9)$On the other hand, it is much harder to coordinate the activities of a large group. $(10)$In addition, large groups may be more likely to encourage social loafing, the tendency of group members to exert less individual effort on the assumption that others in the group will do the work. $(11)$Finally$,$ the quality of group decisionmaking also depends on the cohesiveness of a group. $(12)$When the people in a group like one another and feel committed to the goals of the group, cohesiveness is high. $(13)$ Under these conditions, members may work hard for the group, spurred by high morale. $(14)$But cohesiveness can undermine the quality of group decision$-$making. $(15)$If the group succumbs to groupthink, strong pressure to conform prevents its members from criticizing the emerging group consensus. $(16)$In such a group, amiability and morale supersede judgment. $(17)$Members with doubts may hesitate to express them. $(18)$The result may be disastrous decisions $-$ such as the Bay of Pigs invasion, the Watergate burglary and cover$-$up$,$ or the ill$-$fated Columbia and Challenger space flights.

The relationship of sentence $7$ to sentence $6$ is one of

a$.$ addition.

b$.$ time.

c$.$ illustration.

d$.$ cause and effect.