C. Mechanical, because temperature can cause solid objects to expand and contract, causing rocks to split apart.
The change in velocity (v₂ - v₁) is
<em> (-20) / (the object's mass)</em>.
Call it a crazy hunch, but I can't shake the feeling that there was more
to the question before the part you copied, that mentioned the object's
mass, and its velocity before this force came along.
This question sounds like it came after some activity where
some forces were observed. Since we were not there, and
we don't know what the activity was, we don't know what forces
were observed, and we have no clue to how they might be related
to the motion of the Earth around the sun.
In series circuit, Req = R₁ + R₂ + R₃ + ···
In parallel circuit, 
<h3>Q7.</h3>
total resistance in the upper branch = R₂ + R₃ = R₂ + 2
R₂ + 2 = 12
R₂ = 10Ω
<h3>Q8.</h3>
Req = 1.7Ω
