下列哪个是合法标识符？

B. 123.9

C._2#

D.public

B._implements

C.-implements

D.implements-1

A．-book

B．5files

C．+static

D．-3.14159

B．a_10

C．_123

D．ab

A．123

B．_name

C．class

D．1first

A．123

B．_name

C．class

D．1first

A．_book

B．5files

C．#NAME?

D．-3.14159

B．class

C．int

D．const2

B.3file

C.$good

D.a$2

A、*y

B、a+b

C、a123

D、123