Which of the following reactions proceeds via a secondary free radical?

The reaction of $1,3$-butadiene with $1$ equivalent of $HBr$ at different temperatures yields products $A$ and $B$. Identify the relationship between $A$ and $B$. The reaction scheme is as follows:
$CH_2=CH-CH=CH_2 + HBr (1 \text{ eq}) \xrightarrow{25^{\circ}C} A$
$CH_2=CH-CH=CH_2 + HBr (1 \text{ eq}) \xrightarrow{-80^{\circ}C} B$

Appropriate reducing agent for the following conversion is:
$CH_2=CH-CH_2-CHO \to CH_3-CH_2-CH_2-CH_2OH$

The reaction of $CH_3-CH=C(CH_3)-CH=CH_2$ with $(1) \ O_3$ followed by $(2) \ H_2O, Zn$ gives which products?

How can ethene be produced from ethanol?

In the reaction $C_2H_5CH = CH_2 + H - X \rightarrow$ Product,what is the major product?