What letters replace the question marks in this series?

ABC
?DF
D?L
HP?

After some relaxing, the Seekers of Knowledge received a gift from an anonymous person. The gift was a magical scroll case that had 12 gemstones on it. The Seekers were a bit shocked at the elegance of the scroll case as they knew it was very expensive.

There was a catch with it. They had no idea how to open it. As they looked over the scroll case with amazement, suddenly a magic mouth appeared before them and said, "The code to open this ...

Allen was in the process of sprucing up his large backyard by planting trees, when he came to a tough decision. He had already planted the following types of trees: Khejri, Himalayan Pine, Maple, Llala Palm, and Agave Cactus, when he noticed that he could only plant one more. Given the trees that he had already planted, which of the following type would he choose?

Hard Quandong
Chinese Fir
Eucalyptus
Ulmo
Wild Olive
...

The freezing point of pure water is 32 degrees Fahrenheit (0 degrees Celsius). However, by simply adding a common household ingredient, you then can easily cool the water 20 degrees past the freezing point, and still keep it in liquid form.

How is this possible?