The given figure shows the structure of filamentous blue green algae, Nostoc with a structure marked as X. Select the option which shows the correct identification of the X with its feature.

A. Spores - Reproduction

B. Heterocysts - Nitrogen fixation

C. Pellicle - Recycling of nutrition

D. Mucilaginous sheath - Photosynthesis

Please do not use chat terms. Example: avoid using "grt" instead of "great".