________a language family is a group of languages with a common origin and similar vocabulary, grammar, and sound system.

A．What linguists call

B．It is called by linguists

C．Linguists call it

D．What do linguists call

交流电焊机空载电压不得超过（）伏，焊机把线电缆应采用专用橡套多股细铜线电缆。