拟南芥是一种自花传粉植物，染色体数目为2N=10，是一种特别理想的生物学研究材料。拟南芥作为实验材料的优点不包括[ ]

A．染色体数目少，便于显微镜观察和计数

B．个体较小，适合于实验室大量种植

C．生命周期很短，子代数量多

D．自花传粉，野生型基因高度纯合

As it is used in the passage, the word "photosphere'' (Line 2, Para. 2) means ______.

A．a scarlet gaseous envelope surrounding the Sun

B．the luminous envelope of gas surrounding the Sun

C．the mixture of gasses that surrounds any planet or star

D．the luminous irregular envelope of highly ionized gas outside the chromospheres of the Sun