Lewin's Research Concerning Both How to Promote Economical and Nutritious

Lewin's research concerning both how to promote economical and nutritious eating habits, and what kinds of leaders elicit the best work from group members, was important in establishing which of the following?

A) Different topics require different research strategies.
B) Social psychology could be used to understand and help solve practical problems.
C) Early theories in social psychology were often historically and culturally limited.
D) Social psychologists may unwittingly influence the behavior of research participants.