What is $X$ in the following reaction?

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R).$
Assertion $(A):$ Synthesis of ethyl phenyl ether may be achieved by Williamson synthesis.
Reason $(R):$ Reaction of bromobenzene with sodium ethoxide yields ethyl phenyl ether.
In the light of the above statements,choose the most appropriate answer from the options given below:

Which of the following compounds when heated with $CO$ at $150^{\circ} C$ and $500 \ atm$ pressure in the presence of $BF_3$ forms ethyl propionate?

Diethyl ether can be decomposed by heating with

Give the major product formed by heating $CH_3-CH_2-CH_2-O-C(CH_3)_2-CH_2-CH_3$ with $HI$.

Write the mechanism of the reaction of $HI$ with methoxymethane.