Himalayas is the highest mountain in the world .这样表示对吗

Himalayas is the highest mountain in the world .这样表示对吗
关于下面两组语句表示方法的问题,在喜马拉雅 亚马逊这类山脉河流前,到底用不用the呢?网上怎么表示的都有,迷糊了.
1a
Himalayas is the highest mountain in the world .
1b
The Himalayas is the highest mountain in the world
2a
Amazon is the longest River in the world.
2b
The Amazon River is the longest River in the world.