Residents in a neighborhood determine the approximate number of dandelions in each yard. Then, they treat some yards with a weed-killing substance. The residents want to answer the question “About how many dandelions are in your yard?” to determine the effectiveness of the weed killer. From which group would it be best to take a sample? all residents of the neighborhood only residents who had their yards treated with weed killer only residents who did not have their yards treated with weed killer only residents who still have dandelions in their yards after treatment with weed killer

Aspacecraft can attain a stable orbit 300 kilometers above earth if it reaches a velocity of 7.7 kilometers per second. the formula for a rocket's maximum velocity v in kilometers per second is vequalsminus0.0098tplusc ln upper r, where t is the firing time in seconds, c is the velocity of the exhaust in kilometers per second, and r is the ratio of the mass of the rocket filled with fuel to the mass of the rocket without fuel. find the velocity of a spacecraft whose booster rocket has a mass ratio of 20, an exhaust velocity of 2.1 km/s, and a firing time of 15 s. can the spacecraft achieve a stable orbit 300 km above earth?