If you are referring to stars, the answer would then be pressure from the nuclear reactions
The thermal pressure that pushes outward and against the pull of gravity in a star is caused by the nuclear reactions that is happening within the stars core. A lot of energy is released during these reactions which produce thermal pressure. The pressure then pushes outward.
To develop this problem we will apply the considerations made through the concept of Doppler effect. The Doppler effect is the change in the perceived frequency of any wave movement when the emitter, or focus of waves, and the receiver, or observer, move relative to each other. At first the source is moving towards the observer. Than the perceived frequency at first
![F_1 = F \frac{{343}}{(343-V)}](https://tex.z-dn.net/?f=F_1%20%3D%20F%20%5Cfrac%7B%7B343%7D%7D%7B%28343-V%29%7D)
Where F is the actual frequency and v is the velocity of the ambulance
Now the source is moving away from the observer.
![F_2 = F\frac{343}{(343+V)}](https://tex.z-dn.net/?f=F_2%20%3D%20F%5Cfrac%7B343%7D%7B%28343%2BV%29%7D)
We are also so told the perceived frequency decreases by 11.9%
![F_2 = F_1 - 9.27\% \text{ of } F_1](https://tex.z-dn.net/?f=F_2%20%3D%20F_1%20-%209.27%5C%25%20%5Ctext%7B%20of%20%7D%20F_1)
![F_2 = F_1-0.0927F_1](https://tex.z-dn.net/?f=F_2%20%3D%20F_1-0.0927F_1)
![F_2 = 0.9073F_1](https://tex.z-dn.net/?f=F_2%20%3D%200.9073F_1)
Equating,
![F\frac{343}{(343+V)}= 0.9073(F\frac{343}{(343-V)})](https://tex.z-dn.net/?f=F%5Cfrac%7B343%7D%7B%28343%2BV%29%7D%3D%200.9073%28F%5Cfrac%7B343%7D%7B%28343-V%29%7D%29)
![\frac{1}{(343+V)}= 0.9073\frac{1}{(343-V)}](https://tex.z-dn.net/?f=%5Cfrac%7B1%7D%7B%28343%2BV%29%7D%3D%200.9073%5Cfrac%7B1%7D%7B%28343-V%29%7D)
![0.9073(343+V) = 343-V](https://tex.z-dn.net/?f=0.9073%28343%2BV%29%20%3D%20343-V)
![(0.9073)(343)+(0.9073)V = 343-V](https://tex.z-dn.net/?f=%280.9073%29%28343%29%2B%280.9073%29V%20%3D%20343-V)
![V+0.9073V = 343-(0.9073)(343)](https://tex.z-dn.net/?f=V%2B0.9073V%20%3D%20343-%280.9073%29%28343%29)
Solving for V,
![V = 16.67 m/s](https://tex.z-dn.net/?f=V%20%3D%2016.67%20m%2Fs)
The correct is b: malleable and ductile.