The lab apparatus shown in the image is a test tube holder. Its main function is to securely hold a test tube, especially when heating its contents, to prevent burns and allow for safe manipulat

18. The lab apparatus shown in the image is a test tube holder. Its main function is to securely hold a test tube, especially when heating its contents, to prevent burns and allow for safe manipulation. The correct option is C.

• A. Holding solid substances when heating is typically done with crucible tongs or a deflagrating spoon.
• B. Supporting heating apparatus and equipment is done by a retort stand and clamp or a tripod stand.
• D. Transferring small quantities of solid substances is done with a spatula.

The final answer is $\boxed{\text{C}}$

19. In a light microscope, the diaphragm is the part located within or below the condenser that is used to adjust and regulate the amount of light passing through the condenser to the specimen. This controls the contrast and resolution of the image. The correct option is A.

The final answer is $\boxed{\text{A}}$