Number a is a correct one
Answer D. The pitch is in the frequency.
Answer:
Radio waves have a wavelength between 
and 
While,
X rays have a wavelength between 1m and 10km.
=> It is one of the condition of diffraction that the obstacle (coming in the way) must be comparable with the size of the wavelength.
=> This shows, that radio waves have a wavelength which is comparable with the size of buildings and can really easily diffract through it
=> While, X-rays are big enough to diffract through the wall.
So, if an X-ray technician stands behind a wall during the use of her machine, she will remain safe.
Answer:
F
Explanation:
The average velocity is the ratio of total displacement and time.The average speed is the ratio of total distance and time.Therefore the average speed and average velocity are equal if the total displacement is equal to the total distance that is if the body travels along a straight line without changing the direction.
1). Take a sample of the substance. The sample should be the largest
possible that will allow it to be be easily handled and the following steps
to be performed with it.
(The density doesn't depend on the size of the sample, and every sample
of the same substance has the same density. But using a larger sample
can improve the accuracy of the measurements you make, and therefore
improve the accuracy of the density you derive for the substance.)
2). Ask or measure the mass of the sample.
3). Ask or measure the volume of the sample.
4). Divide the mass by the volume. Their quotient is the density
of the substance.
