Aliasing and the Sampling Theorem Simplified

Which of the following are reasons why aliasing occurs? Select all that apply.

Aliasing means that a continuous-time sinusoid of frequency $\Omega$ appears at a discrete-time frequency $\omega$ that is different than $\omega = \Omega T$ where $T$ is the sampling interval.

A continuous-time sinusoid of frequency $\Omega$ can appear at a discrete-time frequency $\omega = \Omega T + k2\pi$ where $k$ is any integer and $T$ is the sampling interval.

The highest frequency in a continuous-time signal is 15 kHz. Which of the following sampling frequencies will prevent aliasing when sampling this signal? Select all that apply.