The procedure of developing a test, assessing its goodness, developing norms, etc

According to Piaget, a 6-month-old would be expected to be in which of the following stages:

a. primary circular reactions b. secondary circular reactions c. tertiary circular reactions d. preoperational stage

Which is a limitation of experimental research as an approach to knowledge?

(a) subjects are often assigned randomly, which makes statistical analysis difficult (b) subjects are often assigned randomly, so it is difficult to draw cause-and- effect inferences (c) alternative explanations are made less likely (d) the groups are made pretty much equal, to start with (e) there is often an inadequate attempt to have more than one situation in an investigation, even though there are typically many subjects