Final answer:
Sections of countertop meeting at a 45-degree angle are commonly connected with adhesive or screws. Clamps are temporary and not used as the sole connection method.
Step-by-step explanation:
When two sections of countertop that meet at a 45-degree angle are being connected from below, the most common method used is adhesive, screws, or sometimes both. Adhesive helps to provide a seamless connection but lacks structural strength, whereas screws provide a more durable joint. Clamps might be temporarily used to hold the sections together while the adhesive sets or while screws are being installed, but they are not a permanent solution for connecting countertops.