The correct answer is D.
Echo refers to the sound that is heard when a sound wave is reflected back from a surface. An echo sound is made when a sound wave traveled through the air, hit a hard surface and get reflected back. Generally, hard surfaces reflect sound wave while soft surfaces absorb sound waves. Reflected sound waves are used in application of some devices that are used in range and direction finding.
Answer:
50 MHz
Explanation:
The relation between frequency and wavelength is shown below as:

c is the speed of light having value 
Given, Wavelength = 6 m
Thus, Frequency is:



Also,
Also, 1 Hz =
MHz
So,<u> Frequency = 50 MHz.</u>
Answer: B,C,D
Explanation: Who ever reads this have a great day and stay safe :}
Answer:
protons and neutrons
Explanation:
those particles account for 99.99% of mass
Answer: 26.5 mm Hg
Explanation:
The vapor pressure is determined by Clausius Clapeyron equation:

where,
= initial pressure at
= ?
= final pressure at
= 100 mm Hg
= enthalpy of vaporisation = 28.0 kJ/mol =28000 J/mol
R = gas constant = 8.314 J/mole.K
= initial temperature = 
= final temperature =
Now put all the given values in this formula, we get
![\log (\frac{P_1}{100})=\frac{28000}{2.303\times 8.314J/mole.K}[\frac{1}{299.5}-\frac{1}{267.9}]](https://tex.z-dn.net/?f=%5Clog%20%28%5Cfrac%7BP_1%7D%7B100%7D%29%3D%5Cfrac%7B28000%7D%7B2.303%5Ctimes%208.314J%2Fmole.K%7D%5B%5Cfrac%7B1%7D%7B299.5%7D-%5Cfrac%7B1%7D%7B267.9%7D%5D)



Thus the vapor pressure of
in mmHg at 26.5 ∘C is 26.5