B's eight math unit exams were 75, 80, 80, 83, 85, 90, 92 and 95 respectively.

x A = 18×(78+79+8 1+82+84+88+93+95)= 85；

x B = 18×(75+80+80+83+85+90+92+95)= 85。

S2 =18× (49+36+16+9+1+9+64+100) = 35.5.

s B2 = 18×( 100+25+4+25+49+ 100)= 4 1。

∴.x A =。 x B，S A 2 < S B 2

∴ ① Correct, ② Incorrect,

The median of A is 82+842=83 and the mode of B is 80. ∴ ③ ④ Correct.

So the correct serial number is ① ③ ④.