If P is a set of integers and 3 is in P, is every positive multiple of 3 in P
(1)If x is in P，then x+3 is in P.
(2)If x is in p, then x-3 is in p.
A．条件(1)充分，但条件(2)不充分．
B．条件(2)充分，但条件(1)不充分．
C．条件(1)和(2)单独都不充分，但条件(1)和条件(2)联合起来充分．
D．条件(1)充分，条件(2)也充分．
E．条件(1)和条件(2)单独都不充分，条件(1)和条件(2)联合起来也不充分．