Gaseous mixtures:

(A) can only contain isolated atoms
(B) can only contain molecules
(C) must contain both isolated atoms and molecules
(D) are all homogeneous
(E) are all heterogeneous

Is the correct answer, (D), are all homogeneous?

A gas mixture in a container may start heterogeneous but the constant, random motion of the molecules in it will make it homogeneous in a very short time.

To determine the correct answer, let's go through each option and assess its accuracy:

(A) Gaseous mixtures can include isolated atoms. However, gaseous mixtures can also contain molecules, so option (A) is not entirely accurate.

(B) Gaseous mixtures can contain both isolated atoms and molecules. Therefore, option (B) is not entirely accurate.

(C) This option states that gaseous mixtures must contain both isolated atoms and molecules. However, this is not always the case. Gaseous mixtures can include either isolated atoms or molecules, or even a combination of both. Therefore, option (C) is not entirely accurate.

(D) Gaseous mixtures can be homogeneous, meaning that the composition and properties are uniformly distributed throughout. However, gaseous mixtures can also be heterogeneous, in which the composition and properties are not uniformly distributed. Therefore, option (D) is not entirely accurate.

(E) Gaseous mixtures can be heterogeneous, where the composition is not evenly distributed throughout. This means that different regions of the mixture can have different compositions and properties. Therefore, option (E) is correct.

In conclusion, the correct answer is (E), as not all gaseous mixtures are homogeneous, and they can also be heterogeneous.