The underlined word "upbeat" in paragraph 2 probably means ______.

A．mathematicians may provide theories that are incompatible with those already developed by scientists
B．mathematicians may define situation in a way that is incomprehensible to scientists
C．mathematicians may convince scientists that theoretical assumptions are facts
D．scientists may come to believe that axiomatic statements are untrue

A．work to explain real, rather than theoretical or simplified situations
B．know that well-defined problems are often the most difficult to solve
C．are unable to express their data in terms of multiple variables
D．are unwilling to relax the axioms they have developed