Slime moulds $\stackrel{\mathrm{x}}{\to }\boxed{\mathrm{X}}\underset{\mathrm{z}}{\to }$fruiting bodies.

In the above-mentioned flow chart what is represented by X?

Read the following statements w.r.t. diatoms and select the correct option:

(a) In diatoms the cell walls form two thin overlapping shells, which fit together as in soap box.

(b) The walls are embedded with silica and thus the walls are indestructible.