介于钙化的牙本质基质和成牙本质细胞之间有一层基质终生不钙化，称为（）。

A.罩牙本质

B.球间牙本质

C.前期牙本质

D.髓周牙本质

E.成牙本质

When you think of the Internet, you probably think of ".com." Just what do those three letters at the end of a World Wide Web address mean Every computer that hosts data on the Internet has a (71) numerical address. For example, the numerical address for the White House is 198.137.240.100. But since (72) people want to remember long strings of numbers, the (73) was developed. It is a critical part of the Internet’s technical infrastructure, correlates a numerical address to a word. To (74) the White House website, you could type its numerical address into the address box of your web browser. But most people prefer to use "www. whitehouse. gov." In this case, the domain name is (75) .

（75）处填（）。

A．www.

B．www. whitehouse

C．www. whitehouse. gov

D．whitehouse. gov