单选题 A manufacturer has the following figure for the product A Original Data Increase Variable Cost to ＄15 Unit selling 30 30 Unit variable cost 10 15 Fixed cost 3,000 3,000 Budget selling units 300 300 Margin of safety is ______. (a)increase 50 units (b)increase 100 units (c)decrease 50 units (d)decrease 100 units