For a location covering problem to find the minimum number of fire stations that will assure a response time of no more than 5 minutes, you have the following matrix of inter-district travel times (minutes). If Xi is a binary decision variable denoting whether or not the ith district gets a fire station, what is the correct specification for the constraint that there must be at least one fire station that can reach District1 within 5 minutes?

	District1	District2	District3	District4
District1	0	2	10	6
District2	2	0	6	9
District3	10	6	0	5
District4	6	9	5	0

options:

a	X1 1
b	X1 + X2 1
c	X1 + X2 + X3 1
d	X1 + X2 + X4 1
e	X1 + X2 + X3 + X4 1