rharv is right. There is only one empty slot.
I've narrowed it down to 2 possibilities on Crucial website. I want to be sure I'm getting the correct thing. They both look the same to me but have different item numbers. Both same price $31.99. So, what's the difference.

Can anyone help me choose?

Crucial 4GB DDR3L-1600 SODIMM

CT3315709



• Brand: Crucial
• Form Factor: SODIMM
• Total Capacity: 4GB
• Warranty: Limited Lifetime
• Specs: DDR3 PC3-12800 • CL=11 • Unbuffered • NON-ECC • DDR3-1600 • 1.35V •
• Series: Crucial

Or

Crucial 4GB DDR3L-1600 SODIMM

CT3315698



• Brand: Crucial
• Form Factor: SODIMM
• Total Capacity: 4GB
• Warranty: Limited Lifetime
• Specs: DDR3 PC3-12800 • CL=11 • Unbuffered • NON-ECC • DDR3-1600 • 1.35V •
• Series: Crucial


Does the noise in your head bother me ?