Answer: A
Explanation: We know that f=p*n
f=50*300=15000 Hz = 15kHz.
Have a great day! <3
 
        
             
        
        
        
Answer:
I believe the answer is D.
Explanation:
Protons are found inside the nucleus so are neutrons. Electrons are found outside the nucleus.
 
        
                    
             
        
        
        
Answer:
1992 (Early 1990s)
Explanation:
First of all, i would like to define an extrasolar planet as a planet that orbits a start that is not our own.
The first confirmed detections of extrasolar planets occured in the early 1990s (specifically 1992, some say 1995). The name of the first extrasolar planet is widely believed to be called Dimidium or 51 Pegasi b.  
Extrasolar were searched by monitoring stars for slight dimming that might occur as unseen planets pass in front of them.
 
        
             
        
        
        
Answer: (B) There is complete destructive interference between the incoming and reflected waves
Explanation:
For example, if you pluck a guitar the waves will travel back and forth. They consist of nodes and anti-nodes. It is created, when the wave traveling to one side and bounces of the other end and comes back. As it travels to the other side, it is reflected thus, comes back. So standing waves occurs when there is interference. 
When the wave is produced, the points where the string is not moving are called nodes and where they are moving are called anti-nodes. The positions where nodes are produced, destructive interference occurs and where anti-nodes are produced, constructive interference occurs