Apple Inc increased its revenue by 66 percent from 2010 to 2011, going from $65.2 billion to $108.2 billion. However its operating income during this period increased by 84 percent, going from $18.4 billion to $33.8 billion. By comparison, from 2009 to 2010, its revenues increased by 79 percent and its operating income increased by 139 percent. Revenues for 2009 were $36.5 billion and income was $7.7 billion.

a) What concept explains how Apple's net income could rise by 84 percent when its revenue rose only 66 percent?

b) Does the concept identified in requirement a result from fixed cost or variable cost?