The expression of PD-L1 gene in the population of metastatic melanoma patients has a normal distribution with median 18 units and standard deviation 4 units. Answer the following questions: i) What is the mean expression of PD-L1 gene in the population of metastatic melanoma patients? [0.1 point] ii) What is the mode of PD-L1 gene expression in the population of metastatic melanoma patients? [0.1 point] iii) 68% of metastatic melanoma patients have PD-L1 gene expression between what values? [0.2 point] iv) 95% of metastatic melanoma patients have PD-L1 gene expression between what values? [0.2 point]