Practical Sampling

Why is an analog low pass filter normally used prior to sampling a signal? Select all that apply.

2. You are interested in the frequency range $|f| < 500$ Hz and have an analog anti-aliasing filter with stop band $|f| > 750$ Hz. Identify the sampling frequencies that will prevent noise in the $500 < |f| < 750$ Hz range from aliasing into the band $|f| < 500$ Hz. Select all that apply.