40. Quantitative real-time PCR (qPCR) can be used to quantify mRNA levels of genes in different samples. The cycle numbers for the measurement of mRNA expression of gene X in sample A and B are 25 and 30, respectively, and those of GAPDH (a house keeping gene) in sample A and B are 12 and 13, respectively. What is the fold-difference in gene X expression between sample A and B? The PCR efficiencies for gene X and GAPDH are closed to 100%. (A) A is 4-folds higher than B. (B) A is 16-folds higher than B. (C) B is 4-folds higher than A. (D) B is 16-folds higher than A. (E) None of the above