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Sur Ltd. Income Statement Year Ended December 31 2018 2019 2020 Sales $ 732,000 $ 782,000 $ 832,000 Cost of goods sold 439,200 461,380 482,560 Gross profit 292,800 320,620 349,440 Selling expenses 109,800 109,480 124,800 Operating expenses 36,600 46,920 41,600 Operating income 146,400 164,220 183,040 Income tax expense 51,240 57,477 64,064 Net income 95,160 106,743 118,976 Based on the above comparative income statement, what is used as the base to conduct the vertical analysis? Net income. Sales. 2018 sales as a base for all years. 2018 as a base year