A、When it is near the front of its shell. B、When it is trapped. C、When it is tired. D、When it is in warmer waters. A题目问的是鹦鹉螺在什么时

Of all the animals that live in the sea, (26) crustaceans (甲壳纲动物) have the most unusual of reaching maturity. The chambered nautilus (鹦鹉 ) is a perfect example of how this happens.
    Like most crustaceans (卤虫) , these creatures have hard, permanent shells with many sections of crustacean must produce a new section of shell that will be more suitable for its larger body.
    In the beginning stages of growth, the crustacean’s body occupies a small section at back of the shell. As it begins to mature, the body emerges from the back of the shell and bet to (27) take up space closer and closer to the entrance of the shell. It is at this stage of development that the nautilus is especially vulnerable to predators. In order to remain protected the animal must produce a new section of shell in which to grow.
    In order to produce the new section of shell, the nautilus secretes an enzyme which surrounds the entrance of the shell and forms a temporary support for the addition of charged, calcium compounds in the seawater. (28) The process takes several hours, but when it is completed, a new section of shell is available for the nautilus to move into. As the animal moves into the new chamber, the previous chamber is left empty. Over time, the shell of the chambered nautilus can become much larger than the animals itself.
    (29) An adult chambered nautilus contains an average of a hundred sections. Some specimens have been found with several hundred sections and several feet in length. Most of the larger animals (30) were found in the cold waters of the North Atlantic. It is not know how long the chambered nautilus can live, but scientists have calculated from the shell chambers of these larger crustaceans that their shells may have been slowly growing for hundreds of years.
26．What is the main idea of this talk?
27．When is a nautilus vulnerable to predators?
28．How long does it take produce a new section of shell?
29．How many chambers does an adult nautilus usually have?
30．Where were most larger nautiluses found?