19.在一个不透明的口袋中,有标有数字2,3,4除标号外其余均相同的3个小球,从袋中随机地摸取一个小球后然后放回,再随机地摸取一个小球,则两次摸取的小球标号之和为5的概率是$\frac{2}{9}$.
17.下列运算正确是( )
0 284525 284533 284539 284543 284549 284551 284555 284561 284563 284569 284575 284579 284581 284585 284591 284593 284599 284603 284605 284609 284611 284615 284617 284619 284620 284621 284623 284624 284625 284627 284629 284633 284635 284639 284641 284645 284651 284653 284659 284663 284665 284669 284675 284681 284683 284689 284693 284695 284701 284705 284711 284719 366461
| A. | a•a3=a3 | B. | (ab)3=a3b | C. | a8÷a4=a2 | D. | (a3)2=a6 |