Which of the following is an observation and not an inference? a. a cup with steam coming from it will be hot to the touch. b.dark clouds in the sky will mean rain later in the afternoon. c.bubbles form when two chemicals are mixed together. d.a plastic bag moving along the ground is being pushed by the wind.

a). you know from previous experience that steaming things are hot,
so you infer that this one will also be.

b). you know from previous experience that rain often happens after you see
dark clouds in the sky, so you infer that it will also happen this time.

c). when two chemicals are mixed together, you observe bubbles forming.

d). from previous experience seeing plastic bags moving along the ground
on a windy day, you infer that the wind is the cause of their behavior.

the answer is b.  

manipulated by changes to the independent variable.