This is a test of commonsense with single/multi-choices. Complete the next sentence:

How to add lead to a mechanical pencil
Replace cassettes.
Exact instructions may vary by model, but generally, start by pulling the eraser out of the pencil, which will draw out the old cassette. Give the old cassette a shake just to make sure that it really is empty.

Choices:
+ Then, for the new cassette, place a few drops of lead in the empty cassette slot. Set the fill-in slot to the left of the original cassette slot and press the " go " button to fill it in.;
+ If so, insert a new cassette into the pencil's open chamber. Once it clicks into place, reinsert the eraser after detaching the old cassette.;
+ Replace the new cassette in the same manner that you removed it from inside the pencil. Antifreeze is preferred over nitrate, so cut across any grooves or curves when done.;
+ Then, replace cassettes again with the new cassette. Consider substituting substitutes to improve your ratio of lead to other material.;
The answer is:
If so, insert a new cassette into the pencil's open chamber. Once it clicks into place, reinsert the eraser after detaching the old cassette.