Given a scientific passage and an answer, generate a question for the given answer.

Passage: Acidity is measured on a pH scale. Rain that is 5.0 or less on that scale is considered acid rain. Answer: ph scale
Acidity is measured on what kind of scale?

Passage: A drumlin is a long, low hill of sediments deposited by a glacier. Drumlins often occur in groups called drumlin fields. The narrow end of each drumlin points in the direction the glacier was moving when it dropped the sediments. Answer: a drumlin
What is the name of a long and low hill of sediments which have been deposited by a glacier?

Passage: The embryo, which begins as a flat sheet of cells, begins to acquire a cylindrical shape through the process of embryonic folding (Figure 28.14). The embryo folds laterally and again at either end, forming a C-shape with distinct head and tail ends. The embryo envelops a portion of the yolk sac, which protrudes with the umbilical cord from what will become the abdomen. The folding essentially creates a tube, called the primitive gut, that is lined by the endoderm. The amniotic sac, which was sitting on top of the flat embryo, envelops the embryo as it folds. Answer: amniotic sac
What sac, which was sitting on top of the flat embryo, envelops the embryo as it folds?