Why did Germany declare war immediately after Austria-Hungary declared war on Serbia?

Which of the following quotes from the constitution describes the senate? "no person…who shall not have attained to the age of twenty five years, and been seven years a citizen of the united states, and who shall not, when elected, be an inhabitant of that state in which he shall be chosen." "the…shall chose their other officers, and also a president pro tempore, in the absence of the vice president, or when he shall exercise the office of president of the united states.""the…shall be composed of members chosen every second year by the people of the several states.""the…shall choose their speaker and other officers; and shall have the sole power of impeachment."