Evaluate the pH nature of each option.

• A. Toothpaste: Generally basic to neutralize acids in the mouth.
• B. Anti-acid tablet: Basic to neutralize stomach acid.
• C. Sour milk: Contains lactic acid, making it acidic.
• D. Baking powder: Contains sodium bicarbonate, which is basic.

Based on the analysis, sour milk is acidic, not basic.

The correct option is C. Sour milk.

18. The following are quantities that can be calculated. Which one is not?

• A. Volume: Can be calculated (e.g., length $\times$ width $\times$ height).
• B. Mass: Can be measured directly (e.g., with a balance) or calculated (mass = density $\times$ volume).
• C. Area: Can be calculated (e.g., length $\times$ width).
• D. Density: Is calculated (density = mass / volume).

The question asks which one is not a quantity that can be calculated. All listed quantities can be calculated. However, if the question implies "which one is not typically calculated from basic geometric dimensions alone, but is often a fundamental property measured directly," then mass is the most appropriate answer. Volume and Area are directly derived from length measurements, and Density is derived from mass and volume. Mass itself is a fundamental property of matter.

The correct option is B. Mass.

19. Which component of blood is primarily water and transport nutrients, hormones and waste product?

• A. Red blood cells: Primarily transport oxygen.
• B. White blood cells: Involved in the immune response.
• C. Platelets: Involved in blood clotting.
• D. Plasma: The liquid matrix of blood, composed of about 92% water, and it transports dissolved substances such as nutrients, hormones, proteins, and waste products.

Plasma fits the description of being primarily water and transporting nutrients, hormones, and waste products.

The correct option is D. Plasma.