In a controlled scientific experiment,
a.
no variables are kept constant.
b.
one or more variables are kept constant.
c.
only one variable can be kept constant.
d.
all variables are kept constant.

B one or more variables are kept constant.

Explanation:

one or more variables are kept constant. In a controlled scientific experiment, a scientist alters one variable, called the independent variable. The scientist then studies the effects of this alteration on another variable, called the dependent variable.

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