Which word identifies the kind of sentence that contains the main idea?

Biologists recognize five levels of cell organization. the lowest and simplest is the cellular level. organisms here are either unicellular organisms, like protozoa, or colonial organisms. colonial organisms are composed of single-celled individuals that stay together to sustain the life of the whole colony. next on the scale of complexity is the level occupied by tissue. a tissue is a group of similar cells which perform a similar activity. tissues that are organized around a common function together make up an organ, the third level of complexity. the fourth level of organization is the system, formed by a group of organs which together perform a specific bodily process. the fifth and highest level is the organism level, in which body systems work together in a structure capable of independent life. main idea:

Which statement best summarizes this excerpt from leo tolstoy's the death of ivan ilyich? and in imagination he began to recall the best moments of his pleasant life. but strange to say none of those best moments of his pleasant life now seemed at all what they had then seemed—none of them except the first recollections of childhood. there, in childhood, there had been something really pleasant with which it would be possible to live if it could return. but the child who had experienced that happiness existed no longer, it was like a reminiscence of somebody else.

Ineed simplifying this thesis and making it easier to understand for the reader (more concise) "no matter how religious one may be, one’s faith tends to change under unexpected and challenging circumstances."