Medium stars become dwarf stars, large stars become either neutron (pulsar) stars or black holes
(c) is the correct choice.
El Nino (a), Earth's orbit (b), and solar energy output (d) are all "natural" occurrences. You can't do a thing aboutum.
Fossil fuels ... or, more precisely, humanity's use of vast quantities of fossil fuels as a convenient source of huge quantities of energy ... and the subsequent increase of Carbon Dioxide in the planet's atmosphere, is not the result of "natural" processes. It's the result of human efforts to <em>alter and control</em> Nature, through <em>artificial</em> processes.
The period T of a pendulum is given by:

where L is the length of the pendulum while

is the gravitational acceleration.
In the pendulum of the problem, one complete vibration takes exactly 0.200 s, this means its period is

. Using this data, we can solve the previous formula to find L:
Hi!
The energy of the block is 4 m/s
To calculate this, you need to use the equation for
kinetic energy. The block is sliding (i.e. it's moving). If the object is sliding across a level surface, the only energy it has is kinetic energy, because
there is no change in potential energy (which changes with height). So, the mechanical energy will be pure kinetic energy. The equation is the following, derived from the expression for kinetic energy:

Have a nice day!
[two waves] pass a point [every second]... The answer is in the question (B)