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

Input: Consider Input: Passage: In eukaryotic cells, the DNA is kept in the nucleus. The nucleus is surrounded by a double membrane called the nuclear envelope. Within the nucleus is the nucleolus. Answer: nucleus

Output: In eukaryotic cells, where is the dna kept?

Input: Consider Input: Passage: The pioneer species is soon replaced by other populations. Abiotic factors such as soil quality, water, and climate will determine the species that continue the process of succession. Mosses and grasses will be able to grow in the newly created soil. During early succession, plant species like grasses that grow and reproduce quickly will take over the landscape. Over time, these plants improve the soil and a few shrubs can begin to grow. Slowly, the shrubs are replaced by small trees. Small trees then are succeeded by larger trees. Since trees are more successful at competing for resources than shrubs and grasses, a forest may be the end result of primary succession. Answer: primary succession

Output: What's the term for the gradual progression from simple plants to larger more complex ones in an area?

Input: Consider Input: Passage: The fetus is connected to the placenta through the umbilical cord. This is a long tube that contains two arteries and a vein. Blood from the fetus enters the placenta through the umbilical arteries. It exchanges gases and other substances with the mother's blood. Then it travels back to the fetus through the umbilical vein. Answer: placenta
Output: The umbilical cord connects the fetus to what structure?