Germ layer is a primary layer of cells found in the embryo. The three primary germ layers that generate most of the cells in the developing mammalian forelimb are ectoderm, mesoderm and endoderm. Ectoderm forms the outer component of a mammalian body such as skin, hair, etc. Endoderm forms the inner layer that comprises of the linings of digestive system, lungs and thyroid. Mesoderm is the middle layer that forms the skeletal system, heart, etc. Neural tube is generated by the inward folding of the ectoderm. This process is called neurulation. Ectoderm also forms neural crest.
Water at 4 degrees centigrade has the highest density of all other water temperatures. This is why ice (whose temperature is below 0 degrees) floats on very cold water. This phenomenon is significant in the Artic and Antarctic where ice shelves float on cold ocean waters in the poles. Animals that live in these regions (such as seals and polar bears) use these ice shelves as hunting platforms.
I believe the last option is the correct one for the illustration of the single celled organism provides and the environment that it is found living in.
