Here is a question about this article: A few lines of the Paris Metro in France operate on a four-rail power scheme because they run on rubber tyres which run on a pair of narrow roadways made of steel and, in some places, concrete. Since the tyres do not conduct the return current, the two guide rails provided outside the running 'roadways' double up as conductor rails, so at least electrically it is a four-rail scheme. One of the guide rails is bonded to the return conventional running rails situated inside the roadway so a single polarity supply is required. The trains are designed to operate from either polarity of supply, because some lines use reversing loops at one end, causing the train to be reversed during every complete journey. The loop was originally provided to save the original steam locomotives having to 'run around' the rest of the train saving much time. Today, the driver does not have to change ends at termini provided with such a loop, but the time saving is not so significant as it takes almost as long to drive round the loop as it does to change ends. Many of the original loops have been lost as lines were extended.
What is the answer to this question: What was the solution for the return current problem in Paris Metro?
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So... two guide rails

Here is a question about this article: Maternal factors also play a role in the body's immune response. At birth, most of the immunoglobulin present is maternal IgG. Because IgM, IgD, IgE and IgA don't cross the placenta, they are almost undetectable at birth. Some IgA is provided by breast milk. These passively-acquired antibodies can protect the newborn for up to 18 months, but their response is usually short-lived and of low affinity. These antibodies can also produce a negative response. If a child is exposed to the antibody for a particular antigen before being exposed to the antigen itself then the child will produce a dampened response. Passively acquired maternal antibodies can suppress the antibody response to active immunization. Similarly the response of T-cells to vaccination differs in children compared to adults, and vaccines that induce Th1 responses in adults do not readily elicit these same responses in neonates. Between six to nine months after birth, a child's immune system begins to respond more strongly to glycoproteins, but there is usually no marked improvement in their response to polysaccharides until they are at least one year old. This can be the reason for distinct time frames found in vaccination schedules.
What is the answer to this question: At 6 to 9 months, an infant's immune system begins to respond to which proteins?
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So... glycoproteins

Here is a question about this article: Portuguese wines have enjoyed international recognition since the times of the Romans, who associated Portugal with their god Bacchus. Today, the country is known by wine lovers and its wines have won several international prizes. Some of the best Portuguese wines are: Vinho Verde, Vinho Alvarinho, Vinho do Douro, Vinho do Alentejo, Vinho do Dao, Vinho da Bairrada and the sweet: Port Wine, Madeira Wine, the Moscatel from Setubal and Favaios. Port and Madeira are particularly appreciated in a wide range of places around the world.
What is the answer to this question: Which two Portuguese wines are especially enjoyed around the world?
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So...
Port and Madeira