A problem for nurses conducting quasi-experimental or experimental research is that many times a ...

ANS: C
Ensuring all patients receive the standard of care is the best option available; often there is a procedure in place that all subjects can receive, leaving the experimental group to get the treatment under question, or the experimental treatment.
Statistical control of variables can be used for some things, but cannot take the place of assigning treatment and control groups.
In some cases a placebo treatment is considered nothing and would not be approved by the institutional review board.
Using other research designs as often as possible would limit the use of more powerful statistical analysis and make studies not as generalizable.