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

[Q]: Passage: Immune system organs include bone marrow, the thymus gland, the spleen, and the tonsils. Each organ has a different job in the immune system. Answer: immune system
[A]: Bone marrow, the thymus gland, the spleen, and the tonsils are organs in what body system?

[Q]: Passage: Solar energy travels in photons. Photons travel in waves, which make up electromagnetic radiation. Answer: photons
[A]: Solar energy travels in what?

[Q]: Passage: The placenta is a spongy mass of blood vessels. Some of the vessels come from the mother. Some come from the fetus. The placenta is attached to the inside of the mother's uterus. The fetus is connected to the placenta by a tube called the umbilical cord . The cord contains two arteries and a vein. Substances pass back and forth between the mother's and fetus's blood through the placenta and cord. Oxygen and nutrients pass from the mother to the fetus. Carbon dioxide passes from the fetus to the mother. Answer: placenta
[A]:
The fetus is connected to what by a tube called the umbilical cord?