Which of the following is correct?

a) Running two separate experiments is more efficient than running a single two-way design to study the effects of two factors (suppose the total sample size is the same).

b) A two-way design will have larger residual variation than a comparable single-factor experiment because now there are two factors affecting the response.

c) Two-way designs are very useful for studying synergistic effects.

d) Two-way designs require larger sample sizes because there are more combinations of factor levels (the treatments) that must be studied

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