Identify all the keys $($including nested dictionaries$)$ in students $=\{\text{'}$Semester$\_1\text{'}$: $\{\text{'}$Aston$\text{'}$: $20,$ 'Julia': $25\},$ 'Semester$\_2\text{'}$: $\{\text{'}$Ben$\text{'}$: $21,$ 'Lisa': $26\}\}.$

Question $49$ options:

a$)$

Semester$\_1$ and Semester$\_2$

b$)$

$20,25,21,$ and $26$

c$)$

Semester$\_1,$ Semester$\_2,$ Aston, Julia, Ben, and Lisa

d$)$

Aston, Julia, Ben, and Lisa