What did the biology experiments on the Viking landers discover?

a. The soil on Mars has much more microbial life in it than a typical soil on the Earth does.
b. Although Mars's soil is chemically active, it does not contain obviously active micro-organisms.
c. Although there does not appear to be life on Mars today, the soil is full of microscopic fossils.
d. Small, nearly microscopic plants are common in the Martian soil.