I believe the answer is: C. He was worried about his border with North Korea being in jeopardy.
Mao zedong feared that if he let the allied forces took control of north korea, the allied forces would be able to establish a navy base on that territory and exposed the chinese mainland from naval attack from north Korea. Because of this, he decided to help the north korean with both millitary soldiers and supplies.
The name of the river that marked the northern boundary of Islamic expansion in central Asia is Oxus. In 651, Arabs took Iran. After this, they went towards the Oxus river in the north with the intention of defeating the Turkish. The Arabs succeeded and the expansion setling Oxus river as the northern boundary was finally consolidated in 750 C.E.
The correct answer would be the Spanish Flu or influenza
Creating a stronger axis.