Electrical resistance of certain materials,known as superconductors,changes abruptly from a nonzero value to zero as their temperature is lowered below a critical temperature $T_c(0)$. An interesting property of superconductors is that their critical temperature becomes smaller than $T_c(0)$ if they are placed in a magnetic field,i.e.,the critical temperature $T_c(B)$ is a function of the magnetic field strength $B$. The dependence of $T_c(B)$ on $B$ is shown in the figure.
$1.$ In the graphs below,the resistance $R$ of a superconductor is shown as a function of its temperature $T$ for two different magnetic fields $B_1$ (solid line) and $B_2$ (dashed line). If $B_2 > B_1$,which of the following graphs shows the correct variation of $R$ with $T$ in these fields?
$2.$ $A$ superconductor has $T_c(0) = 100 \ K$. When a magnetic field of $7.5 \ T$ is applied,its $T_c$ decreases to $75 \ K$. For this material,one can definitely say that when:
$(A)$ $B = 5 \ T, T_c(B) = 80 \ K$
$(B)$ $B = 5 \ T, 75 \ K < T_c(B) < 100 \ K$
$(C)$ $B = 10 \ T, 75 \ K < T_c < 100 \ K$
$(D)$ $B = 10 \ T, T_c = 70 \ K$
Give the answer for questions $1$ and $2$.

• A
  $(A, B)$
• B
  $(B, C)$
• C
  $(A, D)$
• D
  $(B, D)$