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Answer:
"Sex determination test" is banned in Nepal because many of the women use it as a reason to terminate their pregnancy. Since most of them prefer males as children, whenever they find out that the baby their carrying is a female, they usually abort the pregnancy. They think that women are liabilities to society.
Making the "sex determination test" illegal will prevent the women from tracking the gender of their fetus. Some of them even do it as early as three months of pregnancy. This is because they believe that a son leads the way to heaven for parents.
Thus, this explains the answer.