What Is Implied by the Assumption That Individual People Act

Multiple Choice

What is implied by the assumption that individual people act rationally?  

A)  People think only of themselves and disregard the well-being of others. 
B)  People undertake all those activities that yield benefits to themselves. 
C)  The greater the cost of a charitable deed to a benefactor, the more likely a person is to perform that deed. 
D)  People implicitly calculate the costs and benefits of an activity to decide if it is worthwhile.