Read the passage carefully and choose the correct answer.

 Severe flooding occurred during the 2011 monsoon season in Thailand. The flooding began at the end of July caused by the landfall of Tropical Storm Nock-ten. These floods soon spread through the provinces of northern, northeastern, and central Thailand along the Mekong and Chao Phraya river basins. In October, floodwaters reached the mouth of the Chao Phraya and made parts of Bangkok flooded. Flooding continued in some areas until! mid-J anuary 2012, and resulted in a total of 815 deaths (with 3 missing) and 13.6 million people affected. Sixty-five of Thailand’s 77 provinces were declared flood disaster zones, and over 20,000km* of farmland was damaged. The disaster has been described as “the worst flooding in terms of the amount of water and people affected.”

(https://en.wikipedia.org/wiki/2011Thailand-floods)

What is the cause of the 2011 flooding?