直角三角形ABC中(C为直角)，CD⊥AB，DE⊥AC，DF⊥BC，则＝______.
1
先通读下面的短文，掌握其大意，然后根据短文内容和所给首字母，在空格内填入一个适当的词，使短文意思完整。所填单词在题后横线上必须完整写出。
Wendy tells me something about Halloween in her e-mail. I know a l 1 about Halloween now. People in the USA have a Halloween party on O 2 31st. Children play a game called ‘trick or treat’. They knock on n 3 doors and shout ‘trick or treat’. Usually people give children a treat of some candies.
They wear s 4 costumes and masks. Sometimes they p 5 their faces. It is w 6 .
At Halloween, they have hot drink and eat lots of food. They do not eat mooncakes.
实际中的开关矩阵叫（），接线器的入线接主叫用户接口电路，出线接被叫用户接口电路或各种中继接口电路。
A.接线器
B.集线器
C.用户电路
