Choose the best topic sentence from the four options below.

Choose the best topic sentence from the four options below.
A、The problems of school naturally tend to be less complex than those in the real world.
B、The problems we learn to solve in school are those that someone has already solved.
C、In the real world problems may take days, weeks, or years to solve, or may have no solution at all.
D、One important difference between the problems of school and those of the real world is complexity
【正确答案】：D