To what depth below earth's surface do you have to go before the pressure and temperature cause rocks to melt?
a. at depths of about 500 to 1000 km below earth's surface
b. at the boundary between earth's mantle and core
c. at depths of about 50 to 200 km below earth's surface
d. at any point below earth's crust

Option C, At depths of about 50 to 200 km below Earth's surface

Explanation:

The molten rocks with in the earth is found in the mantle layer. The upper mantle layer starts from 100 kilo meters to 200 hundred kilometers below the earth crust. The temperature and pressure at such depths is extremely high due to which the rocks starts melting. Even if the temperature is low , the excessive pressure leads to liquification of rocks.  

Thus, option C is correct as  from this depth onward the melting of rocks starts.

americans began to notice growing differences between the northern and southern sections of the country. there were more people living in cities in the north, which made it more urban. more people lived on farms in the south, which made it more rural. while many people were farmers in the northern states, there were now other ways to make a living, such as working in mechanical cotton gins factories manufacturing cloth, tools and other goods. people in the north supported high tariffs on goods imported from europe because the extra cost encouraged people to “buy american” instead. in the southern states, planters did not like tariffs because they sold most of their production to europe and bought their tools from there as well. southerners believed the protective tariff was something that only northern factory owners keep their profits high. so the southern politicians came up with an idea. they would opt out. they would simply ignore the protective tariff. it was ‘null and void’ in south carolina. the state would not collect the taxes on goods imported into south carolina, no matter what the president said they had to do. this decision led to the nullification crisis.