∵PC is tangent to⊙ O, and the tangent point is c,

∴∠PCO=90，

At △PCO and △PDO,

Carbon monoxide = DOPO = POPC = palladium,

∴△PCO≌△PDO(SSS)，

∴∠PCO=∠PDO=90，

∴PD is tangent to⊙ O,

Therefore, (1) is correct;

(2) from ( 1): ∠CPB=∠BPD

At △CPB and △DPB,

PC=PD∠CPB=∠DPBPB=PB，

∴△CPB≌△DPB(SAS)，

∴BC=BD，

∴PC=PD=BC=BD，

∴ Tetragonal polychlorinated diphenyl ether is a kind of diamond.

Therefore, (2) correct;

(3) connect communication,

PC = CB，

∴∠CPB=∠CBP，

∵AB is the diameter⊙ O,

∴∠ACB=90，

At △PCO and △BCA,

∠CPO=∠CBPPC=BC∠PCO=∠BCA，

∴△PCO≌△BCA(ASA)，

∴AC=CO，

∴AC=CO=AO，

∴∠COA=60，

∴∠CPO=30，

∴CO= 12PO= 12AB，

∴PO=AB，

Therefore, (3) correct;

(4)∵ Quadrilateral PCBD is a diamond, ∠ CPO = 30,

∴DP=DB, then ∠ DPB = ∠ DBP = 30,

∴∠PDB= 120，

Therefore, (4) correct;

There are four correct numbers,

So choose: a.