Given the following Relational Algebra $($RA$)$ expression E:

RA expression

Note that the above RA expression E corresponds to an SQL query to the database specified in Q$4.$ The SQL query returns the S$\_$ID and C$\_$ID pairs such that the student $($identified by S$\_$ID$)$ is from $\text{'}$CSC$\text{'}$ department, and the student takes the course $($identified by C$\_$ID$)$ with a grade $>=90.$

Here we want to obtain an equivalent RA expression E$\text{'}$ such that E$\text{'}$ is more efficient to implement as in Query Optimization. Your task is to answer the multiple$-$choice questions below to construct the RA expression E$\text{'}$:

In the following multiple choices, the text "sigma" denotes the selection operator sigma, and the text "natural join" denotes the natural join operator natural join

E$\text{'}=$ projection operator$([$Answer $1$ Question $10$

natural join

Answer $2$ Question $10$

Dept $=\text{'}$CSC$\text{'}$

ST $]$ Answer $3$ Question $10$

sigma

$[$Answer $4$ Question $10$

natural join

Answer $5$ Question $10$

Grade$>=90$

EN $])$