Which acid is produced by the bacterium $Acetobacter$ $aceti$?

Match the following bacteria/fungi of Column $I$ with their commercial products of Column $II$.

Column $I$	Column $II$
$A$. Lactobacillus	$p$. Butyric acid
$B$. Aspergillus niger	$q$. Acetic acid
$C$. Acetobacter aceti	$r$. Lactic acid
$D$. Clostridium butylicum	$s$. Citric acid

Select the code for the correct answer from the options given below.

Assertion: Vitamin $B_2$ is found in cereals,green vegetables,brewer's yeast,egg white,milk,and liver.
Reason: It can be commercially produced by some yeasts.

Write the most important characteristic that Aspergillus niger,Clostridium butylicum and Lactobacillus share.

Which bacterium is responsible for the large holes in 'Swiss cheese' due to the production of a large amount of $CO_2$?

The main enzyme obtained from yeast cells is .........