Answer:
![\boxed{\text{B and C }}](https://tex.z-dn.net/?f=%5Cboxed%7B%5Ctext%7BB%20and%20C%20%7D%7D)
Explanation:
In a single-displacement reaction, one element exchanges partners with another element in a compound.
![\textbf{A. } \rm Fe + 2HCl \longrightarrow FeCl_2 + H_2](https://tex.z-dn.net/?f=%5Ctextbf%7BA.%20%7D%20%5Crm%20Fe%20%2B%202HCl%20%5Clongrightarrow%20FeCl_2%20%2B%20H_2)
This is a single-displacement reaction, because the element Fe exchanges partners with H in HCl.
![\textbf{B. } \rm KOH + HNO_3 \longrightarrow H_2O + KNO_3](https://tex.z-dn.net/?f=%5Ctextbf%7BB.%20%7D%20%5Crm%20KOH%20%2B%20HNO_3%20%5Clongrightarrow%20H_2O%20%2B%20KNO_3)
This is not a single-displacement reaction, because it is a reaction between two compounds.
This is a double displacement reaction in which the K⁺ and H⁺ cations change partners with the anions.
![\textbf{C. } \rm Na_2S + 2HCl \longrightarrow 2NaCl + H_2S](https://tex.z-dn.net/?f=%5Ctextbf%7BC.%20%7D%20%5Crm%20Na_2S%20%2B%202HCl%20%5Clongrightarrow%202NaCl%20%2B%20H_2S)
This is not a single-displacement reaction. It is another double displacement reaction, in which the Na⁺ and H⁺ cations change partners with the anions.
![\textbf{D. } \rm Ca + 2HOH \longrightarrow Ca(OH)_2 + H_2](https://tex.z-dn.net/?f=%5Ctextbf%7BD.%20%7D%20%5Crm%20Ca%20%2B%202HOH%20%5Clongrightarrow%20Ca%28OH%29_2%20%2B%20H_2)
This is a single-displacement reaction, because the element Ca exchanges partners with H in H₂O.
are not single-displacement reactions.
The answer is nucleation because can turn liquids into solids.
At constant temperature and pressure, increasing the amount of gas will increase the volume.