Answer:
f = 4 Hz
Explanation:
Given that,
The speed of a wave, v = 12 m/s
The wavelength of a wave, 
We need to find the frequency of the wave. We know that the speed of wave is equal to the product of wavelength and frequency. So,

So, the frequency of the wave is equal to 4 Hz.
Answer:
Technically, humans are causing global warming.
Explanation:
There's some research done into this (sadly not enough), some of which can be found here, https://www.nationalgeographic.com/environment/global-warming/global-warming-causes/ and here, https://climate.nasa.gov/causes/ . The basics are as follows: Earth is a greenhouse. Sunlight comes towards us, and while some is reflected out, most is absorbed through greenhouse gasses, like Carbon Dioxide. Because of humanity, the amount of carbon dioxide in the air has increased, leading to more warmth from the sun to be absorbed. Therefore, the earth is getting warmer because of us.
To solve this problem we will use the concepts of the moment of rotational inertia, angular acceleration and the expression of angular velocity.
The rotational inertia is expressed as follows:

Here,
m = Mass of the object
r = Distance from the rotational axis
The rotational acceleration in terms of translational acceleration is

Here,
a = Acceleration
R = Radius of the circular path of the object
The expression for the rotational speed of the object is

Here,
is the angular displacement of the object
The explanation by which when climbing a mountain uphill is changed to a larger pinion, is because it produces a greater torque but it is necessary to make more pedaling to be able to travel the same distance. Basically every turn results in less rotations of the rear wheel. Said energy that was previously used to move the rotation of the wheel is now distributed in more turns of the pedal. Therefore option a and c are correct.
This would indicate that the correct option is D.
In physics, certain metals elongate when it is heated. This is a consequence of the expansion of the molecules present in a metal tube, for example. This elongation is described by the equation:
ΔL = L0*α*ΔT, where
ΔL is the elongation. In other words, this is the difference between the original length and the elongated length.
L0 is the original leng
α is the coefficient of linear expansion. This is an empirical data for specific kind of materials. For brass, α = 18.9 x 10^6/°C
ΔT is the change in temperature
Rearranging the equation,
ΔL/L0 = α*ΔT, where ΔL/L0 is the percentage of length expansion which is equal to 0.018 (1.8^%)
0.018 = (18.9 x 10^-6)(T-30)
T = 982.4°C