What event directly led to California having a large enough population to apply for statehood in 1850, bypassing the usual territorial stage?

The completion of the transcontinental railroad

The discovery of gold near Sutter's Mill

The establishment of large Spanish missions

The end of the Mexican-American War

Why did Southern leaders in Congress strongly object to admitting California as a 'Free State' in 1850?

They argued that California's population was not yet large enough for statehood.

California refused to pay its share of the war debt from the Mexican-American War.

It threatened to upset the equal balance of power between free and slave states in the Senate.

They believed California's economy would immediately collapse without the institution of slave labor.

The promise that Texas would be divided into five separate slave states in the future

The passage of a new and much more stringent Fugitive Slave Act

The agreement to purchase the island of Cuba and organize it as a slave territory

The removal of all federal troops from Southern forts and territories