For Which of the Following Types of Studies Is It

Multiple Choice

For which of the following types of studies is it impossible to draw cause-and effect conclusions? I) . Completely randomized experiments
II) ) Randomized block experiments
III) . Observational studies

A) I only
B) II only
C) III only
D) It is never possible to draw cause-and-effect conclusions.
E) It is always possible to draw cause-and-effect conclusions.