This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
Amplification of DNA uses the polymerase chain reaction (PCR) technique. Cation used in PCR is: 
Select and explain. 
A. Calcium, B. Lithium, C. Magnesium, D. Sodium
C: Magnesium
Answer: C: Magnesium 
Explanation: PCR is an in vitro method for the polymerase-directed amplification of specific DNA sequences using two oligonucleotide primers that hybridize to opposite strands and flank the region of interest in the target DNA. The specificity and yield in amplifying a paicular DNA fragment by PCR reaction is affected by the proper setting of the reaction parameters (e.g., enzyme, primer, and Mg2+ concentration, as well as the temperature cycling profile). Ref: Feng X., Lin X., Brunicardi F.C. (2010). Chapter 15. Molecular and Genomic Surgery. In F.C. Brunicardi, D.K. Andersen, T.R. Billiar, D.L. Dunn, J.G. Hunter, J.B. Matthews, R.E. Pollock (Eds), Schwaz's Principles of Surgery, 9e.