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What is the meaning of the word checking as it is used in this sentence from the second paragraph of the passage?

Atheme from letter 4 in frankenstein is “friendship is sometimes born in the most unconventional places and situations.” this theme is conveyed through the conversations between walton and frankenstein, and these talks lead them to become confidants and friends. describe a theme from letter 3 of frankenstein and a different theme from letter 4. how are these themes conveyed in each letter? provide evidence from the text to support the themes expressed in each letter.

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: