轮船沿江从A港顺流行驶到B港，比从B港返回A港少用3h，若静水时船速为26km/h，水速为2km/h，则A港和B港相距______km．

During the Renaissance, graphs and diagrams ______.

A．were often used to aid scientific researches

B．served as sources of fresh ideas for some scientists

C．could provide scientists with an immense quantity of knowledge

D．were necessary for the expression of creative ideas