What are plasmodesmata?

Select the correct combination of the statements regarding the characteristics of the middle lamella:
$a$. It holds the different neighbouring cells together.
$b$. It is composed of $Mg$ pectate only.
$c$. It gets dissolved during the ripening of fruits.
Correct statements is/are:

In a plant cell,the deposition of which substance on the primary cell wall leads to the formation of the secondary cell wall?

Cell wall is absent in

Which of the following is absent in the cell wall of a plant?

Which structure keeps adjacent cells in contact with each other?