It would be c the South China Sea
The advances in public and private transportation were one of the key factors of the American urbanization in the first half of the 19th century. Wave of immigrants enter the country which paved way to the rapid increase of factory industries because of the huge labor force they provide.
C. focus .......................................................
There is really nothing to know. All you have to use is context clues and common sense. We know some synonyms for direct are manage and handle. Indirect means not direct, right? And we can't handle or manage earth's interior (core), right? Therefore, the study of earth's interior is an example of indirect observation because we cannot directly manage it.
