You would like a pendulum that swings back and forth once every 2 seconds, but the one you have swings once every 1.9 seconds. which of the following should you do to adjust it so that it has the desired period? remove some mass from the pendulum. make the pendulum slightly shorter. add more mass to the pendulum. make the pendulum slightly longer.

Your pendulum does a complete swing in 1.9 seconds.  You want to SLOW IT DOWN so it takes 2.0 seconds.

Longer pendulums swing slower.

You need to make your pendulum slightly longer.

If your pendulum is hanging by a thread or a thin string, then its speed doesn't depend at all on the weight at the bottom.  You can add weight or cut some off, and it won't change the speed a bit.  

layers of the earth affect the waves and these effects are seen on instruiments such as seimometers

gas and liquid