Answer:


Explanation:
Wave 1, 
Wave 2, 
Wave 3, 
Wave 4, 
The general equation of travelling wave is given by :

The value of
will remain the same if we take phase difference into account.
For first wave,



For second wave,



For the third wave,



For the fourth wave,



It is clear from above calculations that waves 1 and 3 have same time period. Also, wave 2 and 4 have same time period. Hence, this is the required solution.
The correct answer is the Black Market.
The Black Market refers to the informal (and sometimes formal) system of trading of illegal goods by merchants and dealers of products such a weapons, drugs, and good that have been banned by countries. This can also apply to informal marketplaces in countries with sanctions placed upon them get goods that formal treaties are blocking the people from accessing.
Answer:
VR=2
Explanation:
driven=60
driving=30
but VR=driven/driving
VR=60/30
VR=2
hence the velocity ratio VR is 2