Why would a plume of solid silicate rock rising slowly from deep in the mantle begin melting as it neared the base of the lithosphere?
a. the rock heats up and expands at lower pressures, causing it to liquefy
b. temperatures remain high as lowered pressures decrease melting temperatures.
c. the lowered pressures cause rapid heat loss accompanied by melting.
d. none of the above

B. Temperatures remain high as lowered pressures decrease melting temperatures.

Explanation:

Rocks , like ice , contracts on melting so , their melting point decreases on decrease in pressure . When hot solid rocks are pushed towards the surface of the earth, due to decrease in pressure , their melting point decreases , so the solid rocks starts melting without any addition of  heat . Their  present temperature becomes more than their melting point.

russian is not my first language, so i'm afraid i can't write out an answer in russian, but hopefully you can find a way to do so. here's my best translation of the problem: you're given a plot of a particle's position in one-dimensional space over time, and you're asked to find either the total distance traveled by the particle after 6 seconds had elapsed, or the total displacement of the particle after 6 seconds.

if you want the total distance traveled, you would first need to determine the particle's velocity as a function of time, then integrate that over the 6-second interval. we know the position function is quadratic with roots at 0 and 6, so we have

where is unknown. because the position is parabolic, there is symmetry about the midpoint at , and we know that

so that the position function is

which means the velocity is

then the total distance traveled is

so the total distance traveled is 20 m.

on the other hand, if you want to find displacement: the particle starts at and ends at , so its displacement is 0.

i think it would be 5