Question

in deserts and along coastlines, rolling hills of sand can go on for miles. from a distance, these beautiful dunes often resemble mountain ranges or flowing waves. sand dunes offer the perfect setting for an exciting sport: sandboarding.
sandboarding is similar to other board sports, such as surfing and snowboarding. like a snowboard, the sandboard has straps that hold the boarders feet in place and a slick plastic coating on the bottom. to prepare for a run, sandboarders apply slippery wax to the bottom of the boards and strap in. from the top of a dune, sandboarders glide down, making tracks in the sand. because sandboarders do not have to follow twisting mountain trails, they can go straight down a slope and keep picking up speed. but this increase in speed comes with a price: sand feels harder than water or snow when a person falls.
it takes hours of practice to learn how to balance and move on the uneven sand. even the walk back up to the top can be difficult. as sandboarders trudge up the sandy hill, their feet sink into the sand with each step. so, falling on the way back up is just as likely as a stumble on the way down.
when practicing this sport, it is important to have the necessary protective gear. sandboarders must protect their skin by using sunscreen and by wearing long clothing. it is also necessary to have goggles or sunglasses. protective eyewear shields the sandboarders eyes from the bright sun and sparkling sand. even with the proper gear, the sun and heat can be intense. so, it is imperative for sandboarders who are spending a lot of time on the dunes to drink enough water. despite its challenges, sandboarding offers high - speed thrills in beautiful surroundings.
based on clues in the text, why do sandboarders wax their boards?
to help keep their feet firmly planted on the board
to help them control their speed as they race downhill
to help them rock back and forth easily and gracefully
to help them glide down more easily and with greater speed

Explanation:

The text mentions that sandboarders apply slippery wax to the bottom of the boards. Slippery substances reduce friction. Reducing friction on the bottom of the board would make it easier to glide down the sand dune. Also, the text says "from the top of a dune, sandboarders glide down... they can go straight down a slope and keep picking up speed". Waxing (with a slippery wax) would aid in this gliding and speed - increasing process.

Answer:

to help them glide down more easily and with greater speed