I might be wrong but i think is C
Answer:
Cnidarians have groups of similar cells that work together as tissues, while sponges have no tissues, only disconnected regions of specialized cells.
Answer:
Pick Hydrogen, but read below.
Explanation:
The blacks are Carbon. They have 4 connections. The left Carbon has 3 whites connected to it. It is not totally an exclusive answer, but the whites could be anything in Column 17 (F, Cl, Br, I) or Hydrogen. Likely it is hydrogen.