Answer:
<u>C. the types of cells and the tissues in which they can be found in a plant.</u>
You can use dimensional analysis to figure this out.
× 
= 3 grams
If done correctly, ml and ml will cancel out, leaving you with grams.
If you don't know dimensional analysis, simply multiply <em>3 ml</em> by <em>1 grams </em>and that should still get you 3 grams.
The variable that researchers manipulate is the independent variable.
The reason for this is because in experiments different manipulations are often introduced. The sole purpose of these manipulations is to see whether they will affect our independent variable and if they will change its value.