pl explain 18.
Q.18. If 50 heavy DNA ($N^{15}$) are replicated 2 times in ${}^{14}N$ medium, what is obtained?
a) 100 half heavy and half light and 100 light
b) 100 heavy and half light and 100 light
c) 50 heavy and 50 light
d) 200 heavy DNA

Dear student,
Answer is b) 100 heavy and light and 100 light.
If single heavy DNA is replicated with both the strands labelled with N¹⁵​ then after first round of replication two DNA will be formed with both the DNA as hybrid (containing N¹⁵ N¹⁴). In second round of replication four DNA will be formed with two DNA as hybrid (N¹⁵ N¹⁴) and two DNA as light (containing N¹⁴N¹⁴). So if 50 DNA is used initially then 100 DNA will be light and 100 DNA will be half heavy and half light.

Regards