Final answer:
The main ideas are Harriet Tubman's involvement in the Underground Railroad and the heightened dangers due to the Fugitive Slave Act of 1850.
Step-by-step explanation:
The main ideas of Harriet Tubman and the Underground Railroad that stand out in the provided excerpts focus on Tubman's role in aiding the escape of slaves through the Underground Railroad and the increased dangers posed by the Fugitive Slave Act of 1850.
Statement A, which mentions Tubman joining the Underground Railroad to assist black people in reaching states where slavery is illegal, is a central theme as she was a pivotal figure in the network. Statement D, addressing the augmented risk brought by the Fugitive Slave Act of 1850, is accurate as well because it made escaping slavery more perilous, yet Tubman and others persisted in their efforts despite such legislation.