Which of the following is true about the structure and reactivity of steroids?Steroids contain only three fused hydrocarbon ringsSteroids are made up of two isoprene unitsSteroids are nonhydrolyzable lipidsA.III onlyB.I and III onlyC.II and III onlyD.I, II, and III

The correct answer is A. III only.

Here's why:

I. Steroids contain only three fused hydrocarbon rings - This statement is false. Steroids are characterized by four fused hydrocarbon rings, not three.

II. Steroids are made up of two isoprene units - This statement is also false. Steroids are derived from terpenes, which are made up of multiple isoprene units. However, steroids themselves are not made up of just two isoprene units.

III. Steroids are nonhydrolyzable lipids - This statement is true. Steroids, like other lipids, are not able to be broken down (or hydrolyzed) by water. This is a key characteristic of lipids and sets them apart from other biological molecules like proteins and carbohydrates, which can be hydrolyzed.