place a cork in the mouth of the test tube. point the mouth of the test tube away from others. hold the test tube loosely with the fingertips. shake the test tube forcefully to keep contents mixed.
I gave my students this multiple choice question: An important rule for students to know when heating a test tube is to

Only one answer is correct among these 4 choices:
- place a cork in the mouth of the test tube.
- point the mouth of the test tube away from others.
- hold the test tube loosely with the fingertips.
- shake the test tube forcefully to keep contents mixed.

Could you tell me which one is correct?

point the mouth of the test tube away from others.