In today’s fast changing and competitive world, to be able to learn faster and more effectively is not merely desirable; it is ________.

A．essential

B．worthwhile

C．challenging

D．sufficient

计算：（1）6a⁵b⁶c⁴÷（-3a²b³c）÷（2a³b³c³）．

（2）（x-4y）（2x+3y）-（x+2y）（x-y）．