The correct sequence of increasing covalent character is represented by:

1. ${\mathrm{BeCl}}_2<\mathrm{NaCl}<\mathrm{LiCl}$

2. $\mathrm{NaCl}<\mathrm{LiCl}<{\mathrm{BeCl}}_2$

3. ${\mathrm{BeCl}}_2<\mathrm{LiCl}<\mathrm{NaCl}$

4. $\mathrm{LiCl}<\mathrm{NaCl}<{\mathrm{BeCl}}_2$

Which of the following is the electron deficient molecule:

1. C₂H₆ 

2. SiH₄

3. PH₃

4. B₂H₆