What the names of two rock legends that passed away in the early 70s?

 Janis Joplin and Jim Morrison

Explanation:  Janis Joplin (1943 - 1970), American rock and blues singer, distinctly sharp and distinctive voice, with extraordinary vocal abilities. Especially popular during the hippie movement during the 60's of the 20th century. One of the most famous versions of the work she performed was Summertime.

Jim Morrison (1943 - 1971), an American singer, poet and leader of the band The Doors. Known for its performances and an extremely melancholic voice, according to critics, one of the most authentic pop icons of modern music of the 20th century.

offline recommendation systems option b

because recommendation systems produce a list of two ways

1. through collaborative filtering

or

2. through content-based filtering

the answer is definitley b!