27. Computers convert information into binary numbers for processing. Binary is "carry one when two is reached", for example, the binary number 10011 is converted to a decimal number as $1 \times 2^{4}+0 \times 2^{3}+0 \times 2^{2}+1 \times 2^{1}+1 \times 2^{0}=19$, so the binary number $\underbrace{(111 \cdots 111)_{2}}_{2021 \uparrow 1}$ converted to a decimal number is ( ).
A. $2^{2021}$
B. $2^{2021}+1$
C. $2^{2022}-1$
D. $2^{2021}-1$
Answer: D