Which statements about screening and Slater's rules are correct?

AA $2 s$ electron penetrates closer to the nucleus than a $2 p$ electron of the same atom, which is why it is screened less.
BFor a $3 d$ electron the $4 s$ electrons contribute nothing to the screening constant.
CEach electron in the same shell screens by more than an electron from the $(n - 1)$ shell.
DEach $3 d$ electron screens a $4 s$ electron by $0.85$ .

Answer: A, B, D

Penetration

2 s > 2 p

lowers the screening felt by

2 s

. In Slater's rules, groups to the right (like

4 s

for

3 d

) give

0

, and

(n - 1)

electrons such as

3 d

give

0.85

to an

n s

electron. Same-shell electrons give only

0.35

, which is less than

0.85

, so the last statement is false.