Answer:
it may be:
C and D and B the longer the empty space the longer the sound well go
Answer:
Blastocyst is an early embryonic stage in mammals otherwise it is a blastula which is followed by development of gastrula. Blastocyst is a hollow ball like structure that develops by repeated cell division from zygote . It is superficially covered by a layer of cells collectively termed trophoblast.
Explanation:
Vertebrates is the answer