Answer:
Step-by-step explanation:
A ) 90 dB = 9 B 
70 dB = 7 B
If intensity level be I₁ and I₂ respectively 
I₁ / I₂ = 10⁹⁻⁷ 
10² = 100 times
B ) 60 dB = 6 B
If I₃ be the level of intensity of phone ring 
I₁ / I₃ 10⁹⁻⁶
= 1000 times
C ) 
If I₄ be the intensity level of rock concert
I₄ / I₁ = 10¹¹⁻⁹
=100 times.