In PCR amplification why is it important to have a negative control?

What is the purpose of including a PCR reaction with no added DNA? Isn’t this just a waste of expensive enzyme when we know that without template, you will not get any PCR products?

Please help me, this is from practice, double check your answers, previous tutors got it wrong

Explain how reverse-transcriptase PCR is carried out.

Choose the correct statements from the list below. There may be more than one correct statement. A) If you start with 2 DNA templates, after four rounds of PCR you'll have 32 copies B) PCR is useful in making millions or billions of copies of a gene so that it is present in a quantity large enough to study C) quantitative PCR is very similar to PCR, but fluorescent probes are added so that we can measure how much PCR product exists by examining how much the reaction fluoresces D) In real-time reverse transcriptase PCR, the RNA is used as a template to make a cDNA copy (through reverse transcriptase)