Digesting bread is an example of physical change.
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Yes your choices of B C D are right
The dependent variable is plant height.
The <em>dependent variable</em> (plant height) is the <em>property that changes</em> as a result something the scientist does.
The <em>independent variable</em> is the <em>property that the scientist changes</em> systematically (the amount of CO_2) to see its effect.
The <em>number of plants</em> and the <em>types of plants</em> are <em>uncontrolled variables</em>. They may or may not affect the heights of the plants.
H2SO4 + 2KOH --> K2SO4 + 2H2O