Addition of $Br_2$ to $cis-2-butene$ yields ...........

In which of the following reactions will $CO_2$ gas $NOT$ be released?

Consider the following reactions:
$Ph-CH_2-CH=CH_2 \xrightarrow{H^+/H_2O} P$
$Ph-CH_2-CH=CH_2 \xrightarrow[(ii) NaBD_4]{(i) Hg(OAc)_2, H_2O} Q$
$Ph-CH_2-CH=CH_2 \xrightarrow[(ii) H_2O_2/OH^-]{(i) BD_3, THF} R$
Identify the products $P, Q$ and $R$.

When a tertiary alcohol $A$ $(C_4H_{10}O)$ reacts with $20\% \ H_3PO_4$ at $358 \ K$,it gives a compound $B$ $(C_4H_8)$ as a major product. The $IUPAC$ name of the compound $B$ is

In the following sequence of reactions,the alkene affords the compound $B$.
$CH_3-CH=CH-CH_3$ $\xrightarrow{O_3} A$ $\xrightarrow[Zn]{H_2O} B$
The compound $B$ is:

Explain the preparation of alkene from alkyl halide via dehydrohalogenation in detail.