When doing medical research with human subjects, which four limitations are unavoidable? A.)It’s often impossible to repeat trials on the same subjects.
B.)Subjects may report an inaccurate medical history.
C.)It can be difficult to control all possible variables.
D.)It’s impossible to come up with testable scientific questions for human subjects.
E.)There are ethical or privacy concerns to consider.

A, B, C, D

Just a guess, but hope it helps

organisms that lack symmetry are called asymmetrical and belong to phylum porifera/sponges.

organisms with specialized stinging cells belong to phylum cnidaria. stinging cells are called cnidoblast cells.

cnidarians show two morphological forms : polyp (sessile) and medusa (mobile)

filter feeding is observed in porifera members.  

alternation of generation is observed in coelenterata or cnidaria. there is alternation of asexual polyp form and sexual medusoid form. it is also referred as metagenesis.

the titanic would be the correct answer