How does groupthink inhibit the productivity of group work?

How does groupthink inhibit the productivity of group work?
 
  a. It confuses members regarding what the priorities of the group are
  b. It helps those members who feel worried that they will lose good standing in
  the group to feel supported in stating their differing opinions
  c. It may unite group members and encourages the expression of views that
  differ from the views of other members
  d. It may prevent participation of members whose views may be different from
  others or who fear losing good standing in the group by expressing
  disagreement