题目内容：
根据下面资料，回答题 Rare Fossil Record
The preservation of embryos and juveniles is a rare occurrence in the fossil record. The tiny,delicate skeletons are usually scattered by scavengers(食腐动物)weathering before they can be fossilized. Ichthyosaurs (鱼龙) had a higher chance of being preserved than did terrestrial creatures because, as marine animals, they tended to live in environments less subject to erosion. Still, their fossilization required a suite of factors: a slow rate of decay of soft tissues, littlescavenging by otheranimals, a lack of swift currents and waves to jumble and carry away small bones, and fairly rapidburial. Given these factors, some areas have become a treasury of well-preserved ichthyosaur fossils.
The deposits at Holzmaden, Germany, present an interesting case for analysis. The ichthyosaurremains are found in black ,bituminous marine shales(页岩) deposited about 190 million years ago.
Over the years, thousands of specimens of marine reptiles, fish and invertebrates have been recoveredfrom these rocks. The quality of preservation is outstanding, but what is even more impressive is thenumber of ichthyosaur fossils containing preserved embryos.
Ichthyosaurs with embryos have been reported from 6 different levels of the shale in a small areaaround Holzmaden,suggesting that a specific site was used by large numbers of ichthyosaurs repeatedlyover time. The embryos are quite advanced in their physical development; their paddles ,for example, are
already well formed. One specimen is even preserved in the birth canal. In addition, the shalecontains the remains of many newborns that are between 20 and 30 inches long.
Why are there so many pregnant females and young at Holzmaden when they are so rare elsewhere? The quality of preservation is almost unmatched and quarry operations have been carried outcarefully with an awareness of the value of the fossils. But these factors do not account for the interesting question of how there came to be such a concentration of pregnant ichthyosaurs in a particularplace very. close to their time of giving birth.
All of the following are mentioned as factors that encourage fossilization except the_________. A．speed of burying
B．conditions of the water
C．rate at which soft tissues decay
D．cause of death of the animal
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