6. A geometric sequence $\left\{ a _ { n } \right\}$ with all positive terms has the sum of its first 4 terms equal to 15, and $a _ { 5 } = 3 a _ { 3 } + 4 a _ { 1 }$ . Then $a _ { 3 } =$ A. 16 B. 8 C. 4 D. 2
In the ancient Chinese classic ``I Ching'', the concept of ``hexagrams'' is used to describe the changes of all things. They are divided into ``yang'' represented by ``—'' and ``yin'' represented by ``- -''. The figure on the right shows a hexagram. The probability that a randomly selected hexagram has exactly five yang lines is
6. A geometric sequence $\left\{ a _ { n } \right\}$ with all positive terms has the sum of its first 4 terms equal to 15, and $a _ { 5 } = 3 a _ { 3 } + 4 a _ { 1 }$ . Then $a _ { 3 } =$\\
A. 16\\
B. 8\\
C. 4\\
D. 2