Which of these four statements is true about the Tumen River?

Check a map of North Korea and you will see that its north-eastern border is formed by the Tumen River. Most of that border is with China; but a small stretch of the river, the last 17 kilometres (11 miles) before it reaches the Sea of Japan, forms the border between North Korea and Russia. Check a map of Russia and you will see that in the far east there is a 'finger' of territory that juts southwards. This is Primorsky Krai ('the Maritime Province'), that has as its capital the port of Vladivostock. There is a tiny southern salient of Primorsky Krai that meets North Korea at the Tumen river: this is Russia's shortest frontier. Now for the other answers:

The Yangtze River is the deepest in Asia (200 metres or 656 feet).

Beluga sturgeon breed principally in the Caspian Sea.

The Ob River has the longest estuary (1000 kilometres or 600 miles)