C is the correct answer. Double displacement reaction, also called as metathesis, is a type of reaction wherein two compounds react to form new compounds. This case the cations and the anions of the reactants replace each other forming the new compounds.
The first question's answer is
1 is the number of unpaired electrons in the outer subshell of a Cl atom
Use the formula for second order reaction:

C = concentration at time t
C0 = initial conc.
k = rate constant
t = time
1st equation :

2nd Equation:

Find

from 1st equation and put it in 2nd equation:


k = 0.046
A.
everything in existence.