The bread and the butter ( )on sale

are
两个前面都有the说明是两个东西
如果是the bread and butter就是约定俗成的一个组合，用is

are

are
因为The bread and the butter 不是一样东西，所以要用are

are

只要有and连接，不管是否是单复数，都是用are

for example:
Playing football and playing soccer are my favorite sports.

are The bread and the butter 指两个事物
意思为面包和黄油正在销售中。。。
如果原题为The bread and butter() on sale.没有后面那个顶冠词the的话
就应该选is ,The bread and butter指的就是一个事物，即黄油面包