What is the fate of pyruvic acid under anaerobic conditions in our body?

Given below are two statements.
$Statement-I$ - Skeletal muscles usually derive energy by anaerobic respiration.
$Statement-II$ - After vigorous exercise,lactic acid accumulates in muscle fibres,leading to muscle fatigue.
In the light of the above statements,choose the most appropriate answer from the options given below:

During fermentation, alcohol is formed from:

$NADH_2$ generated in glycolysis produces $ATP$ in $ETS$ in the presence of $O_2$. In the absence of $O_2$,this $NADH_2$ functions as:

In animal cells,like muscle,during exercise when $O_{2}$ is inadequate for cellular respiration,pyruvic acid is reduced into lactic acid by

Cramps are formed during vigorous exercise; this is due to the production of . . . . . . in muscles.