We are given the following conditions:

Since we are told it slows down at a constant rate, we know we are dealing with uniformed accelerated motion, or UAM for short.
So we look at our kinematics equation that contains the given variables.

This contains our target variable and our initial conditions. Plug and chug, we get:

Thus the answer is:
a = -7m/s^2
Hope this helps!
Answer:
I believe it's 8.09 seconds, but I'm rusty on my physics.
Explanation:
The equation for solving the time it takes for an object to fall is 
So multiply the distance times 2, and you get 642 meters. Then you divide by gravities acceleration constant, 9.8, and you get 65.51. Finally,
, and you get 8.09 seconds.
I pulled the equation off of wikipedia and I'm unsure if it's the correct one, so hopefully this is correct. :/
Steam is often used as a central heating system when heating many buildings froma central location. So the correct option for the given question is option "D". Steam is actually generated ina single room or place mechanically or by useing fossil fuel or solar energy and then it is passed through pipelines to several rooms and buidings. This kind of system is mostly used when heating several rooms or building at once. Electric heaters are normally avoided as the cost of electricity is far higher than any other source of heating.
Over time, the types of technology can vary and be improved upon so that more advanced techniques become more valued. This could be the situation with mining whereby back in the 1500's in underground mines the rock was broken by fire setting ie lighting a fire below the rock face to heat up the rock and then throwing cold water on it to crack it, so that it could be dug by hand. With the advent of explosives, this all changed so that the rock could be blasted. The increase in advance rates for an underground heading have thus gone from 5-20 feet per month to up to 300meters (984 ft) per month for a 24/7 mining operation, which is a huge improvement.
Frequency is the vibration of noise and the vibration determines the pitch, which we depend on to be a pitch or frequency we can hear. If it's too high or too low our ears can't hear it