Which of the following python command can be used to split $100$ data points, stored in a data frame variable named df$,$ completely into $80\%$ training and $20\%$ validation?

Question $1$ options:

df$\_$train $=$ df $[0$:$80]$

df$\_$test $=$ df$[80$: $100]$

a$.$ df$\_$train $=$ df $[0$:$80]$

df$\_$test $=$ df$[81$: $100]$

None of the answers

df$\_$train $=$ df $[0$:$20]$

df$\_$test $=$ df$[20$: $]$

a$.$ df$\_$train $=$ df $[0$:$80]$

df$\_$test $=$ df$[81$: $101]$